The ESC, apart from receiving signal from the FCB, also powers up the FCB itself.
Connecting 4 ESC's to the FCB translates to providing power (to the to the FCB) through 4 different sources which at times upsets the electronics onboard.
When that happens you will see weird things like ESC's getting extremely hot even at 0 throttle!
Therefore it's advisable to keep just 1 Power source and get rid of the remaining 3.
To do it, just remove the middle Red wire from 3 out of 4 Esc's.
Tip: Don't cut it, Just remove it and heatshrink it like the following video:
BTW, this is NOT something that we do while calibration. It's something that you should do while building your multirotor and let it always stay like that.