Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing the left lane exiting and the other four continuing ahead, and the following text.
Left lane destinations: A408 [Hollow circle diversion symbol] ; Heathrow [Airport symbol]; (T2 & T3) ; Uxbridge.
Right-hand four lanes destinations: Central London, Hounslow M4.
Junction number: 4.
Back when the M4 was built, this was the exit for the whole of what was then called London Airport, and the planners named it Airport Interchange. The whole new-motorway-and-new-airport thing was still very exciting back then.