The imac G4 will handle four tracks. I use one combined with a MOTU 828 and Digital Performer/Audiodesk to record an average of 8-10 tracks with multiple plug ins on each track. The only sluggish part of the system (at least on my 800 MHz) is the “write” time to disk after recording. (A three minute track, for example, might take 15-20 secs.) For less than $1,500 you can choose from the 800 MHz ($999) or the 1GHz ($1,294). Both include the built-in 15” swivel monitor, a cd burner (internal), and somewhere between 256 and 512 RAM depending on the dealer and the offer. You would probably have to add some RAM to improve performance. I believe you can add up to a total of 1 Gb of RAM, although only a portion can be added by the user. Because the iMac is so compact (the base is about the size of a dinner plate) an authorized dealer or repair tech has to handle the maximum RAM upgrade. If you can spend slightly more than $1,500, there is also the 17” iMac G4 1GHz. It goes for $1,594 (MacMall) and includes a built-in Super Drive--Apple’s DVD-R/CD-RW drive. Keep in mind that the imac’s don’t support the use of an additional PCI sound card. They offer Firewire and USB so you would need an external interface.
Now the big question: Is it portable? Yes, compared to the vast majority of desktop computers. It weighs just over 20lbs. Would you rather transport a laptop? Yes. It’s sort of like carrying a very heavy lamp versus a flashlight! But if you only move it once after a recording session, it might not be that big of a deal.
The other option is a laptop. I know the older G3 ibooks can support a limited number of tracks. I used one for a while with cubase to mix from a Fostex VF-16 stand-alone recorder. I would guess that they would be capable of handling four tracks at once, but I have no personal experience using an ibook for that application. The drive speed of the ibook is 5400 and obviously most people suggest something faster (7200) for recording. But it would probably work.
The newer G4 ibooks range from the 800MHz G4($1,094) to the 1GHz ($1,494). They also have built-in cd burners (as opposed to the cd-rom drive in the old G3’s) and either 12” or 14” monitors. The maximum RAM on these machines is 640. Again, if your budget is slightly higher than $1,500, you can get a low-end 1GHz G4 PowerBook for about $1,594 at MacMall.