M3 is the Option key on MacOS X, and the ALT key on most other platforms.
M1 is the COMMAND key on MacOS X, and the CTRL key on most other platforms.
M4 is the CTRL key on MacOS X, and is undefined on other platforms.
M2 is the SHIFT key.
M3 is the Option key on MacOS X, and the ALT key on most other platforms.