Chris, You have said that your rear prop would being going ahead, but your outer props would go astern?( something like that) It sounds like you have your two inboard motors wired to one ESC and your two outboard motors wired to the second ESC, is that correct?
If so, I have had a similar situation. I am using a FrSky with Open TX. I found that is my servo output is at maximum, the control signal from the receiver is too much for the ESC and the ESC, upon reaching full throttle, goes into reverse. I solved this by reducing the servo output range in the transmitter.I would be very surprised if two of your motors maxed out a 320 amp ESC, but then again, Chinese rating are no what they seem.Good luck.Jonathan