Thanks!
Airside - no ciggie
That's incorrect/old information I'm afraid
As of late last year I'm flying out of T4 quite often and there's very much a smoking area airside. Funnily enough, there's no mention of it on the LHR website (in fact, it even says exactly what you did - no smoking airside) but once you're in T4 airside you'll find the smoking terrace right there on the terminal guide.