@mustafagander; "B747 series beautifully over-engineered. One of the design criteria, as I have been informed by those "in the know" was that the aircraft can always make it back to base, no single fault/failure should stop her."
That is the case for most commercial passenger aircraft - even twins nowadays. But what a wonderful aircraft the B747 is. Truly Boeing at its very best, and such a shame that the company has now fallen so low, owing to greed and the desire to make money rather than make good aircraft.
I still occasionally look at the fold-out diagrams of the B747 classic in my copy of 'Handling the big jets' by Davies, and marvel at the engineering and built-in redundancy. Today, though, such over-engineering is not necessary; technology has advanced reliability to the stage where two engines are good for most operations. The 747 is effectively two aircraft within one airframe:- double sets of main gears, double the number of engines, double the number of hyd pumps, etc, etc. As someone mentioned, the fourth engine is only really needed for take-off at high weights, and the same probably goes for the body main gears.
But nevertheless, an awesome aircraft and a game changer; deservedly in the aviation hall of fame.