Of course you can record tracks 3 & 4!!!
telboy- your bleethrough probs sound like you don't have your pan controls set properly - let's simplify and see if this works.
First, just for the sake of simplicity, don't record 2 things at once. Try this:
Plug your drum machine or whatever you are using for click into channel 1 input. Make sure the buss/track selector is set to trk 1. Turn the pan knob for ch.1 all the way left - record. Plug your mic or your bass DI into channel 1 (yes, you can use channel 1 to lay all four tracks) select trk/buss 2, turn pan all the way right - record. Plub your mic, or however you are getting the banjo to line level, into channel 1, select track 3, pan hard left again- record. Plug whatever is next into channel 1, select track 4, pan back hard right - record.
When you mix down, you can adjust the pan of any track how you want - the panning during recording just ensures that the signal is getting to the correct track - remeber: odd track numbers - pan left, even track numbers - pan right.
Hope this helps.
Brad