we have a 4 motor drive and when we drive straight it drifts to the left how do we fix this
Test all motors again
Look over coding
Test all connectors
Make sure there is not more friction on one side versus the other.
If you could loosen the shafts from the motors, give the robot a push and see if it drifts. Some bearing blocks may be tighter than others.
Then motors come into play - is one set to speed inside? Sometimes there are variations in motor ability.
Do you have encoders on each side? You can adjust until the left/right error is 0 when driving straight. Gets hard in a turn.
Shaft encoders are a good idea. If you have your drive chained together, you can just put one on each side, and monitor the rotations very easily.
Drive encoders + PID. We had same issue last year. By chance do you use ports 1 and 10 for wheels.(and/or an Xdrive). They seem to respond less consistently (in my opinion). If you go down the PID route look at some of the post about optimizing the settings. The default will slow things down significantly.