One thing to consider: if you plan to use any "pseudo animations" (as example of this is the awesome Light of My Life) in your game by using RenPy's ATL features, like subtle panning around or zooming in/out to give the still images some life, then you'll need images that are larger than screen resolution. one easy way to acheive that is to render at > screen res. However, 4K would be huge overkill in this case - 20% oversize is plenty.
Another thing: many creators suggest rendering in 4K resolution but at a low number of iterations and then scaling down to 1080. This allows "grain" to be largely removed by the scaling process averaging 4 pixels into one, while still being almost as fast as rendering at 1920x1080.