I also agree that you don't need to pull 4G for a loop. For a nice loop in a small plane, between 3 and 3.5 is enough.
Now for commercial jets...
I knew the barrel roll during the first flight of the 707, but I've also heard Concorde pilots saying that they did the same during line training (no pax o/b unfortunately). The instructor would do one to the left and the trainee would then do one to the right. That was quite a long time ago, I don't know if they still do it...huh... if they used to do it until last year.
