For people theorising that Nintendo may release their Wii U games in 4K on NX, keep in mind that it's not simply a matter of whether the GPU had the raw computational grunt necessary. In fact, if the OP is correct then the ALU performance (i.e. Gflops) shouldn't be an issue for 4K Wii U remasters. What would be a major bottleneck, though, is memory bandwidth. The considerable majority of memory bandwidth in a console is consumed by framebuffer accesses, so if you increase the size of your framebuffer four-fold, you'll want to have close to four times the memory bandwidth available to accommodate it.
The Wii U's eDRAM pool, which developers use to store the framebuffer, reportedly has 70GB/s of bandwidth. So, for Smash Bros, which runs at 1080p/60fps, you would have to assume that that 70GB/s is pretty much fully saturated, as the game's pushing twice as many pixels as most Wii U titles. Hence, if Nintendo wanted to release a 4K version of Smash Bros on NX, they'd want to ensure they had at least four times the framebuffer bandwidth the Wii U had, 280GB/s, plus enough bandwidth for non-framebuffer memory access (i.e. texture reads, etc.). For reference, that's more bandwidth than a GTX 980.
Now, modern AMD GPUs (from GCN 1.2 onwards) have color buffer compression, which provides for more efficient bandwidth usage than Wii U would have had. They claim a 40% overall reduction in bandwidth usage, so you'd be looking at 168GB/s plus non-buffer accesses, so probably over 200GB/s in all. That's quite a big jump over PS4 (which has about 140GB/s usable bandwidth), but just about doable with GDDR5X, if Nintendo want to spend the money on it.
Smash Bros would be the least of Nintendo's worries, though. Almost all of their other games (Mario Kart 8, Splatoon, Mario 3D World, etc.) are rendered at 720p, and assuming any of them comes close to saturating Wii U's 70GB/s of eDRAM bandwidth (not impossible with deferred shading used in a number of those games), then the bandwidth required to run them at 4K starts to become a bit ridiculous. Nintendo would need 378GB/s of framebuffer bandwidth (taking into account color buffer compression and not including non-buffer memory access) to be comfortably able to run these games at 4K.
Now, from a purely theoretical perspective, that's not actually impossible for a late 2016 console to achieve. Either 4096-bit HBM or 384-bit GDDR5X would be able to achieve the necessary bandwidth, both of which would be available to Nintendo within the necessary timeframe. Nintendo have spent money on exotic memory before (1T-SRAM, eDRAM, FCRAM, etc.), so maybe they decide they want to go with HBM for NX, but in order to spend the money on HBM they'd have to spend less on other components, most notably the SoC containing the CPU & GPU. Effectively, Nintendo would have to design a console that is less capable at 1080p in order to accommodate a small number of 4K games.
Given that Nintendo's most successful recent console was a bet against higher resolutions, it would be bizarre for them to suddenly decide that their best course of action is to go gung-ho into the world of 4K. Now, it's not impossible that a small number of 2D or indie games may be able to run at 4K on NX, a PS4-level machine would certainly be able to accommodate that (in fact I'd be confident in saying that Rayman Legends probably would have run at 4K on PS4 had Sony allowed games to output at that resolution), but Wii U games in 4K are an exceedingly unlikely proposition.