Yamaha could be more specific about the SUB 3 and SUB 4 signals in the specs. Looking at the service manual, SUB 3 and SUB 4, each, are MONO signals. Thus, I would expect these jacks in AUX mode to form a stereo pair, i.e., LEFT via SUB 3 and RIGHT via SUB 4.
One could verify by placing SUB 3 and 4 into AUX mode and send a stereo signal out. Pan full left then full right. Hear what happens... I'd try it, but it's getting late, here.
Of course, I could be all wet. Neither SUB 1/2 or SUB 3/4 have sum-to-mono (L+R) switching.
Hope this helps -- pj
P.S. 1/4" TS to 3.5mm stereo cables are quite common (so-called break-out cables, thinking in the other direction). My spouse thinks I have too many such cables.