I have a nonfunctional GTK 4 port at https://gitlab.gnome.org/GNOME/gnome-sudoku/-/tree/mcatanzaro/gtk4. Currently it builds and runs, but the sudoku grid is not shown. I need to investigate further to figure out why the grid is not shown.
I belatedly discovered that @arnaudb also has a GTK 4 port at https://gitlab.gnome.org/GNOME/gnome-sudoku/-/tree/arnaudb/wip/gtk4. Shame, because I have duplicated much of the same work. Anyway, this port currently crashes on startup. I haven't investigated why yet, so I'm not sure if it ever worked, or if perhaps something in GTK 4 changed and introduced new crashes. If it shows a sudoku grid when the crash is fixed, then maybe this branch would be a better bet than mine. However, it would need some work to adapt for 4c6370ee, which moved a lot of code around.
Both branches will need to be adapted for !35 (merged), but this should be a much smaller change.
@arnaudb, any opinions?
P.S. I notice you have a WIP port for Chess as well, but I had already finished mine and merged it into master before I noticed yours. Sorry. :( Either I didn't know about your work, or I forgot if I ever did know....