RCexl discontinued that BEC version of the opto-kill switch like three or four years ago. None of the current RCexl Opto switches have a BEC in them. It isn't shown on the RCexl website either.  So I seriously doubt that you  have a BEC inside your RCexl Opto-kill switch.

I would use either a voltage regulator in series with the ignition power wires on the opto-kill switch or run a 4.8 volt battery pack for it,  I am actually using a Castle Creations voltage regulator with a LiPo pack in one plane with the RCexl opto switch. In a couple of other planes I use the Tech Aero combination BEC and opto kill switch and it works well too.