Heathrow Terminal 4 Station
- Southern Perimeter Road, TW6 3AA
Not all Piccadilly Line trains call here; the service splits just after Hatton Cross, with some trains (indicated "to Heathrow Terminal 5") continuing on through Terminals 1/2/3 to Terminal 5, and others (indicated "to Heathrow Terminals 1, 2, and 3") going to Terminal 4 then looping round to Terminals 1/2/3 and then returning to Central London (see this diagram on Wikipedia). Passengers for Terminal 4 should either take a train indicated to Terminals 1/2/3 in the first place, or change at Hatton Cross. (Note that at least as of early 2008, announcements on the train suggest changing at Hounslow West; this is also a perfectly acceptable place to change, but Hatton Cross is closer to the airport, and there are buses from there to the airport in the case of service failure.)
There's a bus station too, and the Heathrow Connect service from Paddington terminates here. According to Wikipedia, passengers transferring between Terminals 1/2/3 and Terminal 4 can use the Heathrow Connect service free of charge.