Question: why are you using such massive image files? 2D games are very system resource intensive, especially when using high resolutions. You will be severely limiting who can play/run your game if you intend to make a 2D game @ 4K resolutions.
1920x1080 (1080p) should be your target for high resolution/detailed artwork. or something like 1280x720 (720p) if you intend to also release for mobile/tablet platforms. or 480x270 if you are planning on doing low resolution pixel art & want it to scale nicely on all 16:9 aspect ratio resolutions.
Quick note: most common used resolution according to steam stats is 1920x1080, the percentage of all other used resolutions is tiny in comparison. & the most used aspect ratio is 16:9. 4K doesn't even come into the picture at all because the amount of 4K users is tiny because not many people have the money to build a machine capable of running games smoothly at 4K. 2K on the other hand is getting more popular, 1920x1080 is still the most dominant resolution.
3840 x 2160 -- 4K (2% approx)