Hi, Try direct soldering on motor 4 and test fly.
Copied from troubleshooting, ardupilot.org
My copter flies well, but then dips a motor arm in a fidgety manner while hovering
The most likely cause of this is your bullet connectors. After your initial setup, when you test motor direction and electrical connections, westrongly recommend that you cut off the bullet connectors and directly solder the motors wires to the ESC wires. That way is vibration-proof and will lead to much more reliable operation.
If your wires are soldered without bullet connectors and you still see this issue, there are some other things that can cause this problem. Your motor may be going bad: either the bearings are going or your shaft is horribly bent. A motor with bad bearings takes more power to spin. In that situation, the ESC could be cutting out to protect itself. Or it maybe flying slow enough to stall the motor. Attach a current sensor between your battery and the motor and test the difference between a good motor and the bad one. If you are seeing higher draw in the bad one, replace or fix it.