BAKFlip MX4 vs UnderCover Ultra Flex: The Drier Bed Decides It

Buy the UnderCover Ultra Flex. On paper it matches the BAKFlip MX4 line for line: 400 lb evenly distributed, a 5-year warranty for the original buyer, a clamp-on install with no drilling on most trucks, and a tailgate that opens and closes with the cover shut. On Amazon they cost the same on six of the eleven same-truck, same-bed listing pairs we compared, and the gap on the other five is small either way. So the choice comes down to water and the fold, and the Ultra Flex takes both. Our scores put it clearly ahead.

We rate the MX4's leak risk high and the Ultra Flex's moderate. MX4s let water in most often at the front corners by the rear window, and a warranty replacement doesn't reliably cure it. Half the Ultra Flex owners who mention water report a dry bed, and no leak spot repeats. Folded, the MX4's stack leans back toward the rear window, while the Ultra Flex stands upright on its prop rods, clear of the cab and glass. Neither keeps your view out the back when it's open.

The MX4 still has its buyers. It's the one with a rack-ready version (the MX4 TS) and a lift-assist version (the MX4 EZ-Lift), and on a 3rd gen Tundra it's the one with a confirmed 6 ft 7 in listing. If none of that applies to your truck, take the Ultra Flex.

The short version

Why the Ultra Flex gets our pick:

• Water: it keeps the bed dry more often, and when water does get in, it's a little in one spot or one kind of weather. The MX4 leaks more than a cover at its price should, and in predictable places.
• The fold: the folded panels stand upright on prop rods instead of leaning toward the glass. That's what sells people on the UnderCover.
• Durability: both have long records on earlier covers, but recent MX4s draw more trouble on arrival: rails that won't seat, clamps that don't stay tight, broken or misinstalled latches.
• Underside: earlier Ultra Flexes have a carpeted underside that owners like, and RealTruck now describes a padded one on the Gen 3. The MX4's underside dents.

What the MX4 does better:

• Versions: the MX4 TS adds T-slot side rails for a rack, and the MX4 EZ-Lift adds a lift assist. RealTruck's Ultra Flex page says nothing about racks.
• Fit: a confirmed Tundra 6 ft 7 in listing. Its Amazon listings also read through 2026 on every truck still in production, while the Ultra Flex listings we confirmed stop at 2025, though RealTruck lists the same Ultra Flex parts for 2026 trucks. On a 2026 truck, confirm the year with the Ultra Flex seller.

What's the same: the 400 lb rating, the 5-year warranty that ends when you sell the truck, a tailgate that works with the cover down, no keyed lock and no published weight. RealTruck sells both and quotes 30 minutes or less for each install.

Side by side on paper

Here are the two from RealTruck's own product pages and, where those are silent, the Amazon listings:

Both are hard tri-folds whose panels sit nearly flush with the bed rails, both fold forward for the whole bed and let you drive with the cover open, and neither has a published weight. Both are sold today as a Gen 3, and neither Gen 3 has been out long enough to build a record of its own. The Ultra Flex Amazon listings still describe the earlier carpeted underside, so ask the seller which version is in the box.

The matching ratings mean less than they look. 400 lb is weight spread evenly across the whole closed cover, on both. It isn't a rating for a person kneeling on one panel or the corner of a loaded cooler, and neither maker publishes a point-load figure. Both warranties are limited to the original buyer, so a used cover of either kind comes with none, and RealTruck says the Ultra Flex has to be free of snow and ice before you open it.

How each one folds

This is the difference you'll see every time you open the bed. Both hold the folded stack on prop rods. The Ultra Flex's rods hold its panels upright, so nothing touches the cab or the glass. The MX4's stack leans back toward the rear window. RealTruck says it leaves the third brake light clear, and on an F-150 it clears the bed camera, but on a Colorado it blocks the view out the back entirely, and on a Tacoma the rods wouldn't seat with the cover folded all the way up and the stack pressed on the rear camera.

Don't buy the Ultra Flex for your rear view, though. Standing right behind the cab, its stack still blocks much of the view out the back window, and on a 2020 truck with a camera rear-view mirror it covers that camera too. Plan on driving on your side mirrors with either one folded. What the Ultra Flex saves you is panels resting against the cab and glass.

The fold costs some bed. By one owner's estimate the Ultra Flex stack stands about 8 in into the bed. On a Maverick's 54.4 in bed that's roughly 15 percent of the floor length while the cover is open; on a 67.1 in F-150 short bed it's about 12 percent. We have no comparable figure for the MX4, but the lean has a payoff: a Ram owner keeps a leaky MX4 because folding it against the glass is what makes room for an ATV. Measure your tallest load against the front of the bed before you choose.

The tailgate doesn't separate these two. The MX4's dual-action tailgate seal lets the gate swing under a closed cover, and most Ultra Flex listings describe the same kind of seal. Neither has a keyed lock, so with either one shut the bed is as secure as your tailgate lock. Lock the tailgate.

Water: the Ultra Flex is drier

This is where the scores split. The MX4 takes on water first at the front corners next to the rear window, and a front seal that doesn't fully overlap the bulkhead foam will drip right there. The tailgate end comes a close second and the side rails third, and hard rain is the usual trigger. A sloped driveway can send pooled water toward one front corner. Don't count on a warranty swap to fix it, since replacements have leaked just as badly as the originals.

The Ultra Flex is a water-resistant cover, and a good one. Half its owners who mention water report a dry bed, through torrential rain, a first big storm and an automatic car wash. The rest describe a little at the front of the bed, some in very hard rain or wind, or water by the tailgate in a car wash. One dealer-installed cover started leaking between the folding panels in year 3. No spot repeats, so there's no single weak point to chase.

The MX4 can be made a lot drier. Silicone along the rails, weather stripping under the rail seal and at the bulkhead, a tailgate seal kit and careful alignment have taken MX4s to a dry bed, and some arrive dry. Route both drain tubes all the way out of the bed and keep them clear; they clog and pop off. On a 4th gen Tacoma the bed's drain hole is 5/16 in and the MX4's tube is 5/8 in, so you drill or fit a reducer, and the install guide governs any drilling. RealTruck's Ultra Flex page doesn't describe its drain setup, so follow the install guide for your truck.

Whichever you buy, run a hose over the closed cover one zone at a time before the first storm and open the bed after each pass to find the wet spots. If the bed has to stay bone dry, neither is the answer; a camper shell is.

Build, finish and how they age

The Ultra Flex has the steadier record, and the MX4 the shakier recent one. Earlier covers of both have lasted. MX4s have run five to nine years, and an Ultra Flex has run nine and come through baseball-sized hail that wrecked the truck's aluminum body. None of those long reports describes a current Gen 3 of either cover.

• MX4 on arrival: the redesigned rails are harder to seat, clamps don't always stay tight, and recent covers have arrived scratched, with a broken latch or with latches installed wrong at the factory.
• Ultra Flex fit: covers that won't lie flush, with gaps at the sides or the tailgate, are its most common complaint, and seals can arrive slightly pinched. A gap that survives adjustment is a call to UnderCover, not a job for foam.
• Finish: the MX4's matte top scratches easily and dents if you chop packed snow and ice off it, even with a snow broom. The Ultra Flex's aluminum skin can dent too. If you want a top that shrugs off scratches, that's the Armor Flex in the picks.
• Hail: one MX4 was trashed in a violent summer storm. The nine-year Ultra Flex came through hail that shattered the truck's sunroof. One cover each proves little, but the Ultra Flex has the better story.
• Winter weight: in hard-winter climates the MX4's panels can take on water and get heavy to fold; RealTruck says the EZ-Lift cuts lift force to 10.5 lb. The Ultra Flex's panels are built the same way, aluminum over a polymer core, and we have no freeze-thaw report on them either way.
• Small parts: the MX4's prop rods pop out and its plastic latch parts get brittle with age, with replacement parts slow to arrive. The Ultra Flex has release cables on both sides of the bed, so you can open it from whichever side you're standing on.

Inspect either cover the day it arrives, before you install it. Check the hardware against the parts list for your truck, look over the seals and the finish, and keep any request to the maker in writing.

Install, racks and tool boxes

Both clamp on with no drilling for the cover itself on most trucks. The MX4's trick is the clamps: each one has to click up into a groove in the rail before the rails sit flush. Then set it front to back so the front seal sits fully on the bulkhead foam and the rear gasket rests on the tailgate. The Gen 3 Ultra Flex uses RealTruck's Tip-and-Click mounts, and owner times run from 30 minutes to about 90 for a first install. On either one, look along both sides and across the tailgate before the first rain.

Racks and tool boxes split them. For crossbars or a rack on the cover, the MX4 TS is the version with T-slot side rails for it; RealTruck's Ultra Flex page doesn't mention racks, so ask before you plan one. BAK's own BAK Box no longer fits the redesigned MX4 rails. A swing-out toolbox runs under an Ultra Flex for one owner, but RealTruck says the Ram 1500 (DT) 5 ft 7 in Ultra Flex for trucks without the multifunction tailgate isn't compatible with UnderCover's own SwingCase, so check your part.

Which trucks have both

RealTruck sells one part number per truck and bed for each cover, and Amazon lists them the same way. We confirmed the listings below on each one's own product page; the fitment tables under the picks carry the part numbers, bed lengths in inches and each listing's restrictions.

• F-150 (Gen14, 2021 on, and Gen13, 2015-2020): both have confirmed 5 ft 7 in listings for each generation and a Gen13 8 ft 2 in listing. The Ultra Flex adds the Gen14 6 ft 7 in and 8 ft beds and the Gen13 6 ft 7 in. The Gen14 MX4 listing reads 2021-2026 and the Ultra Flex ones 2021-2025, though RealTruck's pages list the Gen14 Ultra Flex parts for 2026.
• Silverado and Sierra 1500 (T1XX, 2019 on): both have 5 ft 10 in and 6 ft 7 in listings that say they work with the MultiPro and Flex tailgates. The MX4 short-bed listing says it won't fit the CarbonPro composite bed; RealTruck lists the Ultra Flex short-bed part without the CarbonPro bed and says it won't work with factory bed storage boxes.
• Ram 1500 (DT): check the tailgate first. Both have a confirmed 5 ft 7 in listing for trucks without the multifunction split tailgate and a 6 ft 4 in listing for trucks with it. Only the Ultra Flex has confirmed listings for the other two combinations and a 2019-2022 RamBox listing that needs the silver cargo rails, not the black track system. RealTruck sells MX4 versions for those too; match the exact listing.
• Tacoma (4th gen, 2024 on, and 3rd gen, 2016-2023): both have a confirmed 4th gen 5 ft listing, and RealTruck says both installs require the Toyota deck rail system. Only the Ultra Flex has a confirmed 4th gen 6 ft 2 in listing. For the 3rd gen 5 ft 1 in bed, both have listings: the MX4's is built around the factory track system, and for the Ultra Flex the Amazon listing and RealTruck disagree about the bedside storage boxes. Only the Ultra Flex has a confirmed 3rd gen 6 ft 2 in listing.
• Tundra (3rd gen, 2022 on): the bed decides it. We confirmed the MX4 for the 6 ft 7 in bed and the Ultra Flex for the 5 ft 7 in bed; the Ultra Flex listing is for trucks without the Trail Special Edition storage boxes. Check fit for your truck.
• Colorado/Canyon and Maverick: both have confirmed 2023-on Colorado and Canyon 5 ft 2 in listings and Maverick 4 ft 6 in listings. The MX4 Maverick listing reads 2021-2026, though the truck started in 2022.
• Ranger (6th gen): neither is confirmed. An MX4 listing exists but its product page didn't show the fit clearly, and RealTruck lists an Ultra Flex part we didn't find on Amazon. Check fit for your truck with the searches in the picks.

Which one for how you use the truck

• Daily driver, gear in the bed most days: the Ultra Flex. Same rating and warranty, a drier bed out of the box, and panels that don't rest on your glass when you drive with it open.
• Hard rain or weekly car washes: the Ultra Flex, and check the tailgate end after the first few washes. If you buy the MX4 anyway, budget an afternoon and a tailgate seal kit.
• A rack on the cover: the MX4 TS, the version of either cover built for one.
• Snow country, parked outside: the Ultra Flex for sealing, or the MX4 EZ-Lift if heavy panels are the worry. With either, brush the snow off before you open it rather than chopping ice off the top.
• Loading cargo across the closed top: neither. Look at the Armor Flex, the Ultra Flex with a coated top, and spread the weight.
• A Tundra with the 6 ft 7 in bed: the MX4, the one with a confirmed listing for that bed.

The Ultra Flex, unless you need the MX4 TS

For most trucks we'd buy the Ultra Flex. It costs what the MX4 costs on most listings, carries the same rating and warranty, keeps the bed drier, and folds upright off the glass instead of leaning on it. Ask the seller whether the box holds the Gen 3, check the gaps on day one and lock your tailgate.

Buy the MX4 for the TS rack rails, the EZ-Lift assist or a bed only it has a confirmed listing for. Inspect it the day it arrives, set the rails square, route the drain tubes out of the bed and plan on some seal work.