The four tiny X-Racer Quadrant ESCs can be installed separately on the arms, but also can be installed together like a PCB under the flight controller. The X-Racer ESCs can be considered as 4-in-1 ESCs, but not the same to other 4-in-1 ESCs that only can be used on one PCB. One disadvantage of other 4-in-1 ESCs is, if one part of the ESC damaged, then the whole PCB have to be changed. It would be too much trouble to the pilots. But there will be no problem with X-Racer ESCs. The four tiny ESCs can be used together or seperately. Even one is damaged, you just need to change one, no need to change the whole PCB.