BAKFlip MX4 vs UnderCover Ultra Flex: Which Upright Hard Folder Wins?

The BAKFlip MX4 vs UnderCover Ultra Flex decision comes down to a short list of differences, because the two covers are so alike. For most owners we’d buy the BAKFlip MX4: it publishes a 400 lb rating, has a rack-ready version and has by far the larger owner base. The UnderCover Ultra Flex is the better choice if you carry tall loads and want to shut the tailgate on them, if you have a Ram with the Multi-Function tailgate, or if it’s cheaper for your truck.

Both are hard aluminum folders that stand the folded stack upright behind the cab, both can be driven fully open, and both clamp on without drilling and come with a 5-year warranty. We haven’t tested either one ourselves. This comparison uses the maker copy on Amazon, etrailer’s reproduction of BAK’s spec sheet, owner ratings and the listings we checked on November 12, 2026. Where a figure is a maker claim, we say so.

Quick comparison

Bed access and everyday use

On paper the two are a tie: each says 100% bed access, and each lets you drive with the cover fully open. Our bed access calculator allows about 6 in at the cab for an upright stack on both, which leaves roughly 61 in of the F-150’s 67 in box open.

The differences show up in daily use:

• The MX4 folds forward in sections and stands the stack on prop rods. BAK says you can drive fully open.
• The Ultra Flex stands upright on dual prop rods without resting on the back of the cab, which should spare the cab paint. UnderCover lists three secure positions: closed, partly closed and fully open.
• Tailgate with the cover up. UnderCover says its dual-action tailgate seal lets you close the tailgate with the cover up or down. That helps if you carry something tall at the front of the bed and want the gate shut behind it. We didn’t find the same claim in BAK’s specs for the MX4.
• Rear window. Upright panels block it on both covers, so expect to rely on mirrors and the camera.

If you rarely haul anything taller than the bed rails, none of this matters. If you do it weekly, the tailgate seal is the one genuine usability edge in this comparison.

Security and locking

Neither cover has a key. The MX4’s patented latches engage automatically as each panel closes, and the Ultra Flex is secured through your tailgate lock. On our three security levels both are level 2: hard panels that stop casual theft, held shut by a locked tailgate. If someone gets the tailgate open, both covers open too.

So security is a tie, and the practical fix is the same for both. If your tailgate doesn’t lock, add a Pop & Lock tailgate lock. If tools live in the bed, our most secure tonneau covers guide covers keyed alternatives.

Load rating

This is the clearest spec gap. etrailer’s copy of BAK’s spec sheet gives the MX4 a 400 lb rating, evenly distributed. The Ultra Flex listing publishes no load rating at all. That doesn’t mean it can’t carry weight, but you have nothing from the maker to rely on, so don’t stack gear or stand on it. Treat either cover as a lid, not a deck. If you need a high rating on the cover, the Extang Solid Fold ALX lists 600 lb in our hard-folder roundup.

Weather sealing

We found no published water test, from the makers or independent, for either cover. The MX4 uses edge-to-edge rubber seals and a built-in drainage system with drain tubes, so route the tubes through the bed’s drain holes and keep them clear. The Ultra Flex’s headline sealing feature is the dual-action tailgate seal; the listing gives no more detail than that.

The tailgate gap is a common leak point on folders, so a seal designed to work with the cover up or down is a sensible feature, but it isn’t proof the Ultra Flex stays drier. No cover we track is truly waterproof. See are tonneau covers waterproof? for what to expect.

Installation

Both are no-drill, clamp-on installs. BAK claims a one-person install in under 30 minutes for the MX4, which is a maker claim. UnderCover gives no time for the Ultra Flex, and its F-150 listing shows a 70 lb weight, so a second person makes placing it easier. Plan on longer than the claim the first time either way. Hard folders clamp to the exact shape of your rails, so the right part number matters more than the brand. Our install guide covers squaring the rails and setting the seals.

Bed racks and accessories

BAK sells the MX4 TS, a version with T-slot rails for racks, and etrailer confirms the TruXedo Elevate rack fits the MX4. It also warns that the Thule TracRac SR can interfere with the MX4’s seals. For the Ultra Flex we found no maker-confirmed rack pairing, so this one is a gap if you plan to carry kayaks, ladders or a rooftop tent. More options are in tonneau covers with a bed rack.

The Ultra Flex has a carpeted underside, which is a small nicety for cargo that touches the panels from above. Whichever you buy, a bed mat protects the floor, and the Husky bed mat suits the F-150 5.5 ft bed. See the accessories page for more.

Prices for the same trucks

Both are sold per truck and bed, and we only compare listings that fit the same bed. These are the Amazon prices we saw on November 12, 2026:

The “MX4 is $100 cheaper” line is true on the most popular fitment, the F-150 5.5 ft, but it isn’t the pattern across trucks: on three of these five, the Ultra Flex was cheaper. The gaps are small enough that a coupon or a shipping difference could flip them. Prices move often, so check both listings: MX4 price, Ultra Flex price. We don’t quote sale prices because we can’t verify them as they change.

Fitment: which one covers your truck

Fitment is where the two diverge most.

• Ram 1500. The standard MX4 listing (5’7” bed) doesn’t fit the Multi-Function (split) tailgate, and RamBox trucks need the RB version. The Ultra Flex has a 6’4” listing, part UX32013, made for the Multi-Function tailgate (without RamBox). Our Ram 1500 guide walks through the split-tailgate problem.
• Silverado/Sierra. The MX4 listing we checked is for the 5’10” bed and the Ultra Flex listing is for the 6’7” bed, so they aren’t a like-for-like pair in our data. Both note that they work with MultiPro/Flex tailgates.
• Tacoma, Ranger. We checked the MX4 for the 2016–2023 and 2024+ Tacoma and for the Ranger, but the Ultra Flex only for the 2016–2023 Tacoma.
• Model years. The Ultra Flex listings we checked end at 2025, while the MX4’s run to 2027 on most trucks. If you own a 2026 truck, confirm fitment with the listing’s tool before you order.

Not sure which bed you have? Use the bed size finder, then check the part number for your truck against the listing.

Owner track record

The MX4 has the bigger owner base by a wide margin: 763 ratings averaging 4.2 on the F-150 5.5 ft listing, 1,839 on the Silverado listing and 1,285 on the Ram, per the Amazon listings we checked. The Ultra Flex had 46 ratings averaging 4.4 on the F-150 listing and 238 averaging 4.3 on the Silverado listing. A higher average on fewer ratings tells you less, but neither cover shows a red flag. The MX4 has a longer public history, and its newer versions are covered in the MX4 review.

Who should skip each one

• Skip the MX4 if closing the tailgate over a tall load is a regular need (UnderCover claims it, BAK’s specs we found don’t), or if your Ram has the Multi-Function tailgate.
• Skip the Ultra Flex if a published load rating matters, if you plan to put a rack on it or if you want the longest owner record.
• Skip both if you need a cover that locks with its own key, or if budget is the main concern. The Gator EFX costs about half as much and opens about two-thirds of the bed; see Gator EFX vs BAKFlip G2.

BAKFlip MX4 vs UnderCover Ultra Flex: which should you buy?

• Most owners: the MX4. Same warranty and access, a published 400 lb rating, a rack-ready version and the bigger track record.
• You haul tall cargo and want the tailgate shut: the Ultra Flex, for its dual-action tailgate seal.
• Ram 1500 with the Multi-Function tailgate: the Ultra Flex 6’4” listing.
• You plan a bed rack: the MX4 TS.
• Whichever is cheaper for your truck: on a near-tie like this, price per fitment should break it.

Still weighing hard folders against other designs? Our best hard tri-fold tonneau covers ranks every folder we track, and BAKFlip MX4 vs Revolver X4s compares the MX4 with BAK’s hard roll-up. For F-150 owners, the best tonneau cover for the F-150 covers fitment details, and the cover matcher ranks every line we track against how you use your bed.