Pete, it has to work that way as the RS4 board has no capability to handle the couplers or light.
The RS5 board, although never appearing in stock K-Line stuff, does have provisions for both the coupler and the backup light. It also handles the IR receiver, so the AD20 board that was used in Lionel RS4 wireless tether equipped locomotives was not needed with RS5 or later sound boards. As some point I'd like to try a wireless tether with an upgrade using the RS5 board, maybe my Williams Cab Forward... 
