DanG wrote:

We have done exactly this quite a few times. I suspect he saw a
repair job on large, heavy copper water line. We have used two
rosebuds (there is your 4" diameter flame) to get the moisture out
of the line. To get the copper up to solder temperature we
certainly do use oxy acetylene on 4" heavy wall copper. A B
bottle will work, but I think you would be hard pressed with a
typical propane torch.
....

That was my read of what he must have seen, too....