Update, after debugging: the failure to find a working video mode had nothing to do with the 4K monitor, and everything to do with a problem in our GRUB configuration generator and having /gnu/store on a Btrfs submodule.
In particular, this generated pice of GRUB configuration: --8<---------------cut here---------------start------------->8--- if loadfont /rootfs/gnu/store/m1fx9h7gzw78k0n4da0khbga5i6k8ipk-grub-2.04/share/grub/unicode.pf2; then setup_gfxterm fi --8<---------------cut here---------------end--------------->8--- was causing the setup_gfxterm to not be called due to failing to resolve the unicode.pf2 file when using a Btrfs submodule. The originally attached patches still provide some > simplification/cleanup that would make it good to merge nontheless, I believe! Maxim
