You could try comming from the line out on your soundcard to a stereo (or some kind of amp with a headphone jack on it) for the drummer to listen to while playing along. Are you using the Tascam as a mixer/preamp? You can use the mix out line from the cassette deck to the line in on your soundcard, use the Tascam just like you would if recording on it, just leave the tape paused, this way you can have 4 mics for the drums, set the levels cloce on the Tascam then record on a stereo track on your PC. I hope this makes sense. There is no set rule for recording order, if the guitar, bass and vocals sound all right there is no reason why drum tracks can't be added.