You will need to use 3 channels and tx mixing. One channel to run both the inner engines together using just throttle, and one channel each for the outer engines using throttle and switched rudder. Consider using rudder to reduce the throttle of an engine on the inside of the turn rather than increase throttle on the outside of the turn, because once you get near to or actually reach full throttle the mix to increase throttle will have negligible effect.

H.