We shouldn't count on users of 1920x1080 or 2560x1600. Who will draw such detailed sprites for 10% of gamers. That's simply unreasonable.
I believe 2x is quite enough, though 3x also might be useful. 4x is excessive.
But there are quite an obstacles for implementing this:
1. Geoscape with globe "pixel-by-pixel" rendering will get dramatic fps drop.
2. Screen coordinates which are hard-coded should become web-like values (%, em, px).
3. There should be new sprites/tiles format implemented, which could handle images for different resolutions.
Getting into OpenGL will make this more feasible, but in this case globe rendering have to be rewritten from scratch.
