This forum doesn't scale itself to high resolution displays like other websites do. Maybe you could ask the 0 A.D. Forum people how they managed to work out their scaling.
https://wildfiregames.com/forum/ Maybe it isn't a scaling problem but a default website sizing problem. But this forum is impossible to browse at 4k whereas that forum is perfectly fine, I only rarely encounter websites that are too small to read. This is one of them.