This is a bug in the current version 1.8.4 of PixInsight on Windows. It happens only with high-dpi monitors on some machines, mainly laptops.
Here is the fix:
- From the main menu, select EDIT > Global Preferences.
- On the Preferences window, select the Core UI Resources section on the left panel.
- On the right panel, uncheck the Automatic UI scaling option.
- Depending on your monitor resolution, set the UI scaling factor to something like 1.0, 1.5 or 2.0. For a 4K monitor like yours, I'd use 1.5. The larger the scaling factor, the larger the fonts and icons will be represented on the screen.
- Click the Apply Global icon (blue sphere) or press F6 to apply.
- Exit PixInsight.
- Run PixInsight.
Let me know if this helps.
This problem has already been fixed in the next version 1.8.5 of PixInsight.