I agree it is probably torque. I saw the same thing in my c130.

It is important however to insure the engines are reliable in operation more than matching rpm. If you get reliable operation but the rpm's are over 200 apart, you could move the strong engines inboard and the weaker outboard.

But don't overlean the engines searching for rpm. Also check the throttle opening to insure all are exactly the same position from idle to WOT. This is important also. It is possible that some of the yaw is from slightly different servo time and engine response. You should be able to control this with slow application of power and rudder.

Haven't had the [&o] herk out yet, 40 degrees small craft warnings and winds gusting to 25mph. Where did spring go?

Jim