RONOS, "Two are standard and two are pushers". Do you really mean pushers or opposite rotation?
Why not email George at 4-max and ask him. He actually owns and flies the original Tony Nijhuis Sunderland as featured in RCME.
Sounds like cheap batteries to me and the voltage is collapsing as mentioned by Martin.
I run a Tony Nijhuis Lanc on a single 3S5000 and can get a good 15 minutes flying on a calm day, never had any problems with batteries - 4max supplied.