Heathrow Terminal 4 is a London Underground station at Heathrow Airport on the Heathrow branch of the Piccadilly line. The station is situated in Travelcard Zone 6.
The station opened on 12 April 1986 to serve the then recently opened Heathrow Terminal 4. It is situated on a unidirectional loop tunnel which was constructed between the existing Hatton Cross and Heathrow Terminals 1, 2, 3 stations. The station is one of the few on the London Underground to have only one platform, and is the only one with one-way train service. It is adjacent to Heathrow Terminal 4 railway station used by mainline Heathrow Express railway services.
In-service routing took trains from Hatton Cross to Terminal 4 then Terminals 1, 2, 3 and back to Hatton Cross. On 7 January 2005, both the loop track and the station were closed temporarily in order to allow the construction of a new rail junction to link to the new Heathrow Terminal 5 station. All trains reverted to using the original westbound track from Hatton Cross direct to Heathrow Terminals 1, 2, 3 which was used prior to the opening of Heathrow Terminal 4. For passengers travelling to or from Heathrow Terminal 4, a shuttle bus was provided from Hatton Cross tube station. This situation continued until 17 September 2006 when the loop line and station were reopened following completion of the construction, and with new security and customer announcement technologies in place at the station.
Now that the new Heathrow Terminal 5 station has opened, the service pattern is that every other train arriving at Hatton Cross will run via the Heathrow Terminal 4 loop, whilst the alternate trains will run direct to Heathrow Terminals 1, 2, 3 and then continue down the new branch directly to Heathrow Terminal 5.
Until 2012, free transfer was not possible between terminals, in contrast to the Heathrow Express. In January 2012, free travel was introduced for Oystercard holders between the Heathrow stations on the Piccadilly line. Journeys from Heathrow Terminals 1,2,3 or Heathrow Terminal 5 to Terminal 4 via the Piccadilly line require a change at Hatton Cross, which is not part of the free travel area.