The last few times I've been through LHR (T4 arrival) I didn't see a FastTrack immigration line or any indication they even have one. Arriving on CO4 at 9:35am is less crowded than the earlier flight, though, and the line at was about 15 minutes or less each trip.
Outbound there is a FastTrack line, and I've never seen it longer than the regular line (yet). Leaving on CO5 on a Tuesday mid-morning helps -- other times are probably a lot worse.