Despite what Lerner wrote in his memoir, and despite some slightly contradictory reporting even at the time, I'm pretty confident when i say that it was never four hours long. It ran around three-and-a-half hours at the first performance, and that includes the intermission.
The list of classic Broadway musicals that ran around three-and-a-half hours at their first performances out of town would include Carousel, The King and I, My Fair Lady and Fiddler on the Roof. (There are myths that the first performance of Carousel ran four or four-and-a-half hours and that the second-act ballet ran something like 50 minutes. Nonsense.) I mention this because three-and-a-half hours was not all that unusual. Show Boat, on the other hand, truly does seem to have run a bit over four hours at its first performance in Washington, D.C.
I think part of the reason why the Camelot myth got out there was because the curtain rose significantly late on opening night in Toronto.
I'm linking the ovrtur Trivia & History page on the original production. The note at the top discusses this.