We don't have a lot of info at this time on getting Race The Sun or other Unity games to work in 4k resolution, but we don't believe Unity prevents 4k. Problems may arise due to factors of your particular system configuration rather than because of the engine or game directly.
If you're attempting to use Nvidia DSR i.e. turning on DSR in the Nvidia control panel (in this case 4x for 4K), and then attempting to set that resolution in the game options, you may find that the game immediately reverts to the original resolution instead. Assuming DSR works for you in other games without issue, maybe try this if you haven't already: apply the higher resolution on your desktop before running the game, then select it in-game.
This will work for some, but if it does not work for you, please open a new ticket to let us know about it.