Scratching my head trying to remember......the S4R has the ST4 (and ST4S) heads....right?
AFAIK:
S4R and ST4s are same heads, valves 36/30, short duration intake cams, std 4V exhaust cams.
S4 and ST4 are same head, valves 33/29, short duration intake cams, std 4V exhaust cams.
All of the above are the 'dropped cam' heads; exhaust cams are ~10mm closer to the deck than SBK heads.
Thanks for the speedy reply.