MacBook keyboards have a similar modifier layout to a Space Cadet in some ways: On your left hand, there’s {Cmd, Opt, Ctrl, Fn} in a row. Emacs on MacOS can use all four modifiers, e.g. mapping them to the historical {Ctrl, Meta, Super, Hyper} keys that were in the same place on a Space Cadet.
Note that this is slightly different from PC keyboards. On Mac, Fn as a modifier is handled by the OS and can be remapped (in System Settings or by individual apps like Emacs). On most PC keyboards, Fn causes different signals to be sent to the OS, making it very hard to use it for other purposes than intended.
