The DS4 LED is a common anode RGB setup, so there's no common ground there, more of a common rail. Each of the RGB LEDs ground is turned on/off via a FET. Should be around 4v or so IIRC on that common anode and that should be there any time the controller is on or charging. If that 4v is not there, then look for a missing/damaged component first, or there might be some issue with the PMIC chip, bottom of the board, top/left IC there.