On the Original Prusa MK4, a gearbox alignment is necessary for user-assembled or serviced gearboxes. This procedure is part of the Selftest or can be initiated from LCD menu -> Control -> Gears Calibration.
Once the procedure is initiated, the printer will prompt for the calibration to start. Press "Continue".
Slightly loosen the three bolts on the gearbox cover, by one turn and a half, then unlock and open the idler. When done, press "Continue".
The printer will go through the automatic gearbox alignment. This process can’t be seen from the outside. Once prompted, tighten the three screws in the pattern indicated on the screen, and close the idler.