From Storm's book, "Mind Over Matter - The Images of Pink Floyd", p. 137:

"But, God, it was worth it. Despite freezing our nuts and getting crazy bored waiting for the weather to improve and the sun to come out, the end results were creamy. And the car lights or red flags used to indicate the lines of communication, the talking of profile head to profile head worked well, as did the positioning of the distand cathedral."

Thus, the four lights represent communication, a major theme of TDB.

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"David Gilmour can do more with one note than most guitarists can do with the whole fretboard."

Dave Mustaine (Megadeth)