When you open minecraft in fullscreen mode and set gui scaling to 4, then restart and open it in a somewhat smaller window, and then access the video settings, you get an error in the logs:

[Render thread/ERROR]: Error saving option GUI Scale: DynamicException[Value 4 outside of range [0:3] Optional[4]]

Because this will not save the setting, starting it the next time will revert the setting to "auto" which on my system is equivalent to 6 in fullscreen mode and just not what I want.

The problem seems to arise from the fact that the valid range in which the settings saving code is allowed to save, is determined automatically based on the current window size.

One could deteremine that range based on the maximum supported resolution instead. 

One could also not try to (re)save the setting if the user hasn't touched it in the GUI.

Also there could be a different setting based on window or fullscreen (or even resolution if one wants to get really nice and fancy, although in case of window sizes this could lead to a lot)