There is indeed an option between FullHD and 4K, usually referred as 1440p (2560x1440), but it's only available on PC monitors as TV manufacturers have skipped it.

OT: Since all Nintendo games hit 720p with ease on WiiU and the Switch will be more powerful, there's little reason to doubt that Nintendo can launch all their games at 1080p. Third party games, tho, that's another story.

But, given that the resolution of the Switch screen will be 720p and that we still don't know if the dock does anything besides charging the device and connect it to the TV, it wouldn't suprise me if Switch games on TV were just upscaled to 1080p, not natively rendered at that res.

First of all buy an UHD player to get the prettiest picture out of that screen. Considering the regular PS4 isn't able to play all games in 1080p. The Xone also get quite a few 900p games, I doubt it's going to be different with the Switch. So I guess the main resolution for third party games on the Switch is indeed sub 1080P, I expect every Nintendo exclusive to run in 1080fps though.

Developers aren't going to bother with anything less than 720p. 3rd party that is.

mostly 720p like wii u. There are very even in power.

Regrads power differences between wii u and switch:
Switch will using a custom tegra chip based on x1 or x2.
Benchmarks: Trex 1080p offscreen
Nvidia tegra x1: 120 FPS
Ipad Pro: 163 FPS
unanounced Tegra chip with a codename: 190 FPS (they removed that but it had a benchmark 1-2 weeks ago, google it).
Radeon HD 4850 (Wii U): 150 Fps
HD Radeon 7870 (ps4): 533 FPS


https://gfxbench.com/result.jsp?benchmark=gfx40&test=547&text-filter=&order=median&ff-desktop=true&ff-lmobile=true&ff-smobile=true&os-Android_gl=true&os-custom_gl=true&os-iOS_gl=true&os-iOS_metal=true&os-Linux_gl=true&os-OS_X_gl=true&os-OS_X_metal=true&os-Windows_dx=true&os-Windows_gl=true&pu-dGPU=true&pu-iGPU=true&pu-GPU=true&arch-ARM=true&base=device

540p is the lowest allowed resolution. More than likely by the end of the generation, Switch will be below even that to 480p and under.

Some iOS/Android ports could run in 4K. Other than that, probably 720p to 1080p.

I honestly don't expect the dock will be able to output to a TV at 4k anyway, so even if it had the power to run certain games at 4k it wouldn't.

720p handheld
1080p on tv
4k if its possible

If it can output 4k for movies etc at 30fps then in theory simple games with low demands could be made with 4k output. You could have NES, SNES and N64 games running in 4k upscaled resolutions. Whether Nintendo allows this is another matter but you get $20-30 android boxes that can output in 4k its not a huge deal. It is however difficult if you are releasing very demanding games where every ounce of power is required just to get the engine moving at a decent frame rate at 720p

Its not going to support 4K in any way, so just stop hoping for that right now. Will save you a lot of grief later.