Lucifer
You don't have to have 4g for a loop. The initial pull of g determines the size of the loop. For example a light pull will result in massive loop and the aircraft may run out of thrust (and therefore airspeed) half way up if the engines are not sufficient. For example a 2g pull in a bulldog would result (most probably) in a stall in some sort of vertical attitude. An F18 or something similar could probably loop with about 2g although the manoeuvre may take about 20000ft! The more g you retain makes the manoeuvre smaller, tighter and retains airspeed therefore keeping the aircraft flyable.
Same priciple for fighters or heavies but as the thrust/weight varies considerably I would say you'd need a massive pull from very high speed on a 747 that would probably exceed the g limits. Similarly the pull out would be more critical than the way up as speed is building and gravity is working against you. Therefore I think you might lose an engine or two on the recovery!
