I guess all you need to do is send the main L/R Stereo mix out of your 4 Track, into the input on your computers sound card. Pick up a 2 RCA to 1 1/8" stereo plug cable ($3.00 radio shack), to get the sound from your 4 track to your computer. Plug the 1/8" stereo jack into the line in on your sound card. Use a program like Goldwave or some other freeware wav editor to record the file, and then use the musicmatch to convert it from wav to mp3 (not sure if musicmatch can do this, never used it.)
Emeric