I've been in a similar situation. I'm not positive what kind of unit you're speaking of but I'm assuming its a 4 trak cassette recorder, so I will answer under that assumption. If not, ignore this message.
One other option you can consider, depending what your mixing down to, is an external bounce. If you're mixing to a cassette deck, you can record on all 4 tracks, then bounce externally to your mixdown to 2 tracks on your mixdown unit, then put this tape back into your multi track and record the last 2 tracks. THis will only give you 6 total tracks, but depending on how good your mixdown deck is, this sometimes works better than an internal bounce because you can play your 4 original tracks on a consumer deck to make sure this premix is right. Another thing I use to do with a four track is record the background vocals on the same track with an instument that is not involved at the same time as the background vocals. For me that was usually the lead guitar track. You have to kinda preplan your levels if you do this, otherwise you will have to 'ride the faders' and the eq at final mixdown. But it will take some experimenting, you will also have to decide which tracks you want to bounce, and live with the sound degregation. I use to bounce the rythm guitar and sometimes background vocals or bass guitar. But it usually depends on the song and, like I said which tracks you can live with the sound degregation of the bounce. .....peace..Ray J