You should be able to do the bouncing within the 4 track itself.
For instance, record drums on track 1, bass on track 2, guitar on track 3.
Then bounce those down to track 4.
Then you have tracks 1, 2 and 3 to record on again.

Then you can repeat the process... record keys on track 1, strings on track 2.
Bounce those down to track 3.
Now you have tracks 1 and 2 open again to do vocals.

You should end up with.

Track 1: Drums/Bass/Guitar
Track 2: Keys/Strings
Track 3: Vocals
Track 4: Vocals or another instrument