You never have to install MSYS2 separately to run MyPaint, unless you want to build it yourself. The parts of MSYS2 that are needed are bundled in the installer (and standalone archives).
Unfortunately there is no setting in the preferences to handle high-dpi monitors, but there is a workaround.
You can tell Gtk/Gdk to use a different dpi scaling by setting the environment variable
GDK_SCALE to an appropriate integer (fractions are not supported), in this case 2.
To do this, open notepad as an administrator, enter
and save that file as
If you now run that file instead of mypaint.exe (and change your shortcuts so they point to that file), the gui should be scaled better for a 4k monitor.
If you didn’t use the installer, and instead extracted a standalone archive, the procedure is the same except for the location of the
.bat file (the location would be wherever the extracted archive is located, in the same folder as
mypaint.exe), and that you wouldn’t have to run notepad as an administrator.
Edit: Corrected variable syntax based on toniw’s suggestion. I’m pretty sure I tried it with the equals sign first (closer to the shell syntax I’m used to) but that it didn’t work. Might have been something else…