about all you can really use is the internal harddrive and maybe CDROM and thats about it. the rest is non-interchangable with an Amiga.
G4's are quite *cute* machines, although i do prefer them running linux rather than the butchered apple *nix that comes pre-installed
once you have a decent Linux Distro, you can then emulate miggyness... dosbox... snes... megadrive... everything... quite well indeed!!!!