Analog to digital

[Perndog]

4 track -> computer

4 track -> DAT

4 track -> CD-R

Any of these will give you a digital recording. You'll probably need a computer no matter what in order to send it, either as a .wav or an .mp3. Then your friend does his recording, uses some spiffy audio editing program (I use Sound Forge) to overdub it, and sends the modified file back to you.

Your tracks will be irretrievably mixed together in the conversion though, unless you save them all as separate files.