Okey dokey
Onest of all, I take it your 4chan has 2 x input pairs (L&R on each) and 1 x output, and your 2chan has 1 x input and 1 x output?
If so, I would suggest plugging the ipp odd into the 2chan, then running Y cables from the amp output into both inputs of the 4chan.
If the 2chan is bridgeable you should be doing that, but you will still need both left and right plugged into it for full output.
Without amp deets I'm basing this purely on guesswork. Throw me some model numbers and I'll double check some stz