@adonnen, of what I’ve recently ascertained, GRUB2 doesn’t appear to be capable of scaling whatsoever. Consequently, on 2560x1400px monitor, by default, it’s set to a low resolution equivalent to 200% scaling:
I presume that on a 4K monitor, it would be low enough resolution to be equivalent to 400%, with the problem compounding the higher than the resolution is set, unless it acquiesces eventually and merely renders everything tiny.