The Nintendo Wii U isn't exactly the most powerful game console on the market. A small subsection of Wii U games run natively at 1080p, but the vast majority are rendered at 720p before being upscaled to 1080p. The console doesn't support 4K output at all, and it's clearly too slow to render games at this resolution.
While the console itself is largely limited to 720p and 1080p, the developers behind Cemu - an experimental Wii U emulator for Windows - have found a way to render games at a much higher resolution. The latest version of Cemu, version 1.7.0, supports customizable graphics, allowing users to apply "graphics packs" to existing titles for a range of graphics improvements.
In a sense, the latest version of Cemu has integrated game modding into the emulator, and each of these graphics packs are basically just mods for Wii U games. Graphics packs can be created to increase the resolution of a Wii U game without much hassle, and while these packs can theoretically modify other aspects of a game's graphics, currently available graphics packs are focused on increasing the render resolution from 720p to 4K.
The difference between 720p and 4K is huge for most Wii U titles. Games like Super Mario 3D World go from muddy garbage to beautifully crisp 4K thanks to the latest version of Cemu. It seems that performance in these games at 4K is quite good as well, at least on high-end PC gaming hardware.
Cemu 1.7.0 is currently available to Patreons who pledge $5 or more per month, although the update will be available to all for free on January 17th. It can be a complicated process to rip Wii U games for use in Cemu, but now that the emulator supports 4K in some games, it might be worth the effort.