The T4 arrivals lounge is nothing special IMHO, but then by the time you've waited an age for luggage to come off the carousel at LHR it can come as some respite. It'd be more useful if it was in the baggage hall, but I guess there's a reason why this isn't the case.
Btw, BA plans to open an arrivals lounge at LHR T1 in time for this Christmas.