Actually the default font size on a 4K monitor is already set to even larger than that, 36! (You can see samples on on the
UI preview page.) It normally works fine for 4K users, so there might be something special about your setup or user settings, one or the other. My guess is it's likely a DPI scaling issue with Windows, and although the game tries to tell the OS not to do that, in some cases users might have to open the .exe properties and turn off DPI scaling for it to work properly.
Would need more info to help you here, though, since you're talking about it in the past tense so I'm not sure if you restarted and it was fixed or what xD. You can also attach your run.log file in found in the game directory and that will let us see what it's doing in terms of starting up in a certain resolution/fonts/etc.
(I've moved this to the Support board since it's not a bug.)
