Best practices in design of multiple screen keyboards

I need at least 4 screens for my keyboard.
First will be Unshifted, Second will be Shifted.
However, I am told to be wary of using Alt-Right and Shift-Alt-Right.
Can you suggest why some would be concerned there?
If I do not use Alt-Right, what would you consider next best?
3rd Screen - Ctl-Alt
4th Screen - ???

It could cause conflict with Windows’ shortcuts or other application shortcuts.

You can choose to use AltGr (aka Right Alt) and Shift AltGr for your 3rd and 4th layers.