It’s a piece of cake to record music from a minidisc recorder onto your G4. As a few other people mentioned above, you can’t do it digitally, but that’s no big deal: unless you use a very expensive mic and mixer to record to your minidisc, you won’t notice any loss in sound quality from the analog output of the minidisc. And you’ll end up with digital files once they’re on your hard disk.
I have transferred a lot of minidisc recordings to CD or my hard disk, and I have never noticed any significant loss of sound quality. I’ve tested both with my stereo and it’s hard to hear much difference. Maybe the Mac is better at this than PCs.
Your G4 has a sound-in jack. Your minidisc recorder has a sound-out jack. You simply run a patchcord from the minidisc output to the G4’s input. Alternatively, you can use a USB-based device such as the Griffin iMic. The minidisc plugs into one end of the iMic and the other end goes in your USB port.
You will need audio recording software to record the sound on your Mac’s hard disk. I use Audiocorder, but there are lots of alternatives available, some of them free. If you have iMovie installed (which you probably do as it comes pre-installed on all Macs) you can actually use that to record sound.
You will have to add tracks manually to the sound file, otherwise it’ll all be one big track. Audiocorder lets you do that pretty easily; I’ve never tried it with iMovie. Assuming you’re using OSX on your G4, you should then import the sound files into iTunes, where you can convert them to MP3s if you like or leave them in their original format.
The only tedious part of the process is creating tracks. Some recording programs will create tracks for you automatically when they detect breaks in the sound, but these don’t work well for session recordings where there’s always lots of background noise.
I actually went a different route: I have my minidisc recorder hooked up to my stereo, and I bought a CD player/recorder as part of my stereo system. It has a remote, so I can sit on the couch and add the tracks as I listen while it’s recording direct (analog input only, of course) from minidisc to CD. Then I take that master CD and copy the tracks to iTunes on my Mac, and I can burn CDs or put the tracks on an iPod (once I finally break down and buy one!).
I have unfortunately heard terrible things about the Archos recorders. I’m waiting for Apple to come out with a truly recordable iPod.