I think the point got missed here, somewhere.
You can record to any track from the 5+6 channels (using the on-board bussing) but there aren't actually tracks 5 and 6 on the tape. There are only 4 tracks; on playback these correspond to tracks 1-4, but when recording you can route and number of channels on the mixer to any track on the tape.
I know that's even more confusing, but the important point is that there are only 4 tracks on the tape.
Example of what tracks 5+6 can be used for:
channel 1 - kick mic.
channel 2 - snare mic.
channel 3 - vocal mic
channel 4 - guitar mic
channel 5 - Left drum overhead mic
channel 6 - Right overhead mic
With instruments/mics plugged in this way, you can route the channels on the mixer this way:
1 - 1/2 panned centre
2 - 1/2 panned centre
3 - 3/4 panned hard left (odd)
4 - 3/4 panned hard right (even)
5 - 1/2 panned hard left (odd)
6 - 1/2 panned hard right (even)
What you end up with is this:
Track 1 - drums mixed from channels 1, 2, and 5 (the right drum track in stereo)
track 2 - drums mixed from channels 1, 2, and 6 (the left drum track)
track 3 - the vocals only
track 4 - guitar only
See the progression? The extra channels can let you bring in additional sources to put multiple channels of the mixer on 1 track of the tape.
Chipper had it right. I just had the time to put more into this, probably.
(This is just an example, remember)