Forgot to mention this: Remove 1 motor plug from the 4 in 1, then run it full throttle while holding it down. Do this for each motor until you find the one that has the problem. Then, inspect every part that turns that rotor.

This sounds very much to me like a shaft slipping. Once you find which rotor is causing it, hold the corresponding gear with one hand, and gently but firmly try to turn the rotor with your other hand. See if you can make it slip. If you can, something's not tightened down somewhere. Check everything...