Keg,
I see that you've scored a 424mkIII, so congrats. As I said before, I have one, and I think it's a great machine.
As you have now discovered, the 'buss' on the 424mkIII is basically a 2-buss/stereo L-R type buss. All 8 inputs on the main mixer get mixed down to either left, right, or somewhere in between.
As per Dr Grip, and the manual, the tape tracks 1 & 3 record off of the Left side of the stereo buss, and tape tracks 2 & 4 record off the Right side. That's basically inherent in the design, and it's why you got the recording you got, as described.
The answer you're looking for, I think, is a way to record 4 tape tracks with 4 separate inputs, simultaneously. The way to do this, is to switch the REC FUNCTION to DIRECT, on each track, 1-4.
When in DIRECT mode, each track will record only what it's direct channel input is. Track 1 records input 1, track 2 records input 2, etc, directly.
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The BUSS vs DIRECT recording methods are all described in the Tascam manual, and the manual is not too bad, if you read it.
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Good luck, and again, congrats. I think the 424mkIII is a great machine, and I'm happy for you.