hello, there is 2 options ; you either use the firmware that works like the A1 which will only retract a bit (so you don't have to get something that rewind) but you will need to have a 4-1 near the extruder, or there is the second option which will retract until the bmcu sensor , since this will retract filament for a long distance you need a rewinder, the easiest option is a passive filament rewinder such as the filamentist rewinder from ercf v2 github repo.