T4: BA5xx arrivals (except A330/A340), IB31xx departures (except A330/A340), BA875x
T4S: BA4xx arrivals and departures, IB31xx A330/A340 departures, BA5xx arrivals when operated by A330/A340
Yes, there are also a few Schengen gates in T4S (M gates), whereas non-Schengen gates are R/S/U. There is no longer a lounge in T4S for Schengen departures.
I can't remember whether IB Express to LGW is from T4 or T4S.