B.S.
not really good suggestions--but ideas. I assume you're talking to the boys about this upcoming move-- so DS4 knows DS2 is coming, etc. I would suggest separate bedtimes...DS2 has to go to bed even 15 minutes earlier than DS4--but DS4 would have to be ready (in PJs, etc, by the time DS2 goes down.) THen maybe DS2 would be asleep by the time DS4 got to bed.
Also--by the time this happens, DS4 may be out of this stage of being scared to go to sleep.
Another angle that might work---DS4 might be less afraid since DS2 is in the room with him. ??