I have a 40" cat running 2 880kv motors 2 hobbling 120a esc rated for 38v max.problem is trying to connect a 6s and 4s in series esc wont arm.how can I fix this?
I have no room for bigger batteries

10S full charged is 42v, higher than the ESC wants to see.

Words of wisdom. Be careful running different lipo packs in series. As the capacity can vary significantly, you run the risk of over discharging one pack before the other and damaging it.

Any suggestions besides changing esc's

Add a three legged voltage regulator to feed the correct voltage to the ESC.

Never mind, I was thinking the ESC was supplying too high of voltage to the Receiver, then the voltage regulator would fix that. You need to find an ESC that will handle that kind of voltage.

You could partially charge your batteries so the total is a few volts lower... but see gubbs' point.

Or just use two 4s packs?

Are 2 4s packs equal in size to 1 6s pack? I would need 4 4s packs and don't have much room. 2 6s packs barely fit under canopy