That's a display issue. The output will be in 4k anyway. It happens on Windows if your DPI scaling (screen scale factor) in Windows display settings is higher than 150%. We should modify MadMapper so it detects the physical resolution of the screen, actually MadMapper is showing the OS screen size, which depends on the DPI scaling, but the rendering will happen in the physical output resolution anyway.
That should be fixed in a coming minor update. Thanks for reporting !