unowho
Think you are correct.
As a wee laddie living next to RAF Waddington in the late 70s the four ship Vulcan scrambles were a sight to behold. Three a/c on the runway at the same time in various stages of take-off.
I seem to remember, although it was a while ago, that all four a/c had to be airborne within something less than three minutes from the time of the alert. Will stand corrected on the exact timing though. Consequently I think a four engine start was a must.