Sorry Daryl, was being lazy with the abbreviations but thanks cmaffia. I used the term as I don't have a 644 so am just going on what the manual says. Was hoping someone with a 644 would chip in with first-hand help but maybe the machines aren't so common nowadays. Not sure which bit you don't understand and what level of detail you want - could end up with a very long post if you need everything explaining, but will give it a shot...
Before you go any further though, I would make sure the heads are clean and your tape is in good condition otherwise these muffled sounds will prevent you understanding what's going on, not to mention spoiling your music. Page 42 in the manual shows how & what to clean. If you haven't got a manual, it's on the internet - if you get stuck, give me a shout and I'll email my copy. You should clean it for each session for best results. I used to have a 4-track cassette machine (Yamaha MT4X) and you can get excellent results if you keep it clean and use good tapes.
Try a test recording on all 4 tracks - use a different piece of music on each one, then play them back one at a time to confirm each one sounds crisp & clear and there is no 'leakage' from adjacent tracks.
You used the term 'bounce' in the original post so guess you're ok with what that does... combines (or mixes) one or more tracks onto a destination track. This destination could be a single track or a pair of tracks. You bounced tracks 1, 2 & 3 to track 4, so you're combining everything onto a single track, so when you listen to it (i.e. 'monitor' it) playing back, the result will be a mono signal.
If you play (monitor) a mono sound to both earbuds (or loudspeakers), the sound will appear to come from a single point in space in the middle. You can pan that mono signal left & right but the sound will always appear to come from a single point.
Stereo sounds needs 2 tracks of audio, one for the left, one for the right, and if you play these with the left signal going to the left earbud (or loudspeaker) and the right signal going to the right earbud, the sound will appear to spread out and come from all points between the two.
When you panned tracks 1 & 3 to the middle and track 3 hard left, you're defining where you want those mono tracks to be placed within the stereo spread. If you were listening to the effect with your earbuds, you were 'monitoring in stereo'. If you want to capture that effect onto tape by bouncing/mixing, you need to create a stereo recording so 2 tracks are needed.
Looking at the 644 manual, I'm guessing you're following the bounce example on page 16, but this will only create a mono sound because it bounces to a single track. I couldn't see an example of bouncing to 2 tracks, so not sure if it can be done - it's a bit difficult without the machine in front of me. If it can be done, then obviously you could only use a maximum of 2 tracks as the source, otherwise you'd run out of tracks.
There's another example on page 12 for mixing all 4 tracks onto a stereo 'master', but this needs a separate recorder because there are no spare tracks on the 644. Even mixing 3 tracks to stereo may be impossible within the 644. There may be a way of overwriting one or more of the original tracks during the bounce, but this would be dangerous as you'd only get one shot at doing this!
As you can appreciate, if the 644 only has 4 recordable tracks, you're quite restricted if you want to bounce stereo signals around within the same machine.
So, pending a real 644 owner confirming this, hope this helps.
Phil.