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 the 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. In-service routing took trains from Hatton Cross to Terminal 4 then Terminals 1, 2, 3 and back to Hatton Cross.

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.

In contrast to the Heathrow Express, London Underground services do not provide free transfer between terminals. Passengers using the Piccadilly Line to travel from Terminal 4 to Terminals 1, 2, 3 would need to purchase a ticket, and no direct services exist in the other direction. Airline passengers connecting between terminals at Heathrow can travel free of charge on the Heathrow Connect to Heathrow Central (Terminals 1, 2, 3).