Hmm you can set custom resolutions via the explorer tool.
I don't recommend it though. Your game default will be 4K & will be scaled down from that, which means a lot of wasted system resources for everyone not using a 4k monitor. Also 2D is very system resource intensive.
Also 4K is not standard. 4K is a niche area. Check out steam stats, you will find that the most common resolution used by everyone is 1920x1080, followed by 1366x768 or something similar for laptop users.http://store.steampowered.com/hwsurvey/
1920x1080 (1080p) = 76.41%
3840x2160 (4k) = 0.42%
Just because something is possible, doesn't mean you should bother with it. 1080p is already very demanding when it comes to 2D. If you want to create a 2D game in 4K then you will need to optimize the shit out of it or it will run like garbage on your average target consumers machine.
The sad fact of the matter is that most people don't have up to $4000 to create or buy custom pc with the capabilities of running games all games on ultra settings or on 4K screens. I believe it's mostly only pc enthusiasts with more money than sense that can afford to chuck so much money into building intel machines with dual sli 1080ti or titan x gpu & 60 odd GB of RAM & so on.
Just look at the stats on steam of what the average user is using. It's quite surprising.