I think there is generally a bit of a public perception problem on the "4 engines are safer than 2" concept, particularly when flying over the Atlantic.
It is pretty fair to say that the chance of both engines on a twin failing at the same time for mechanical reasons is effecitvely zero. If the chance of even a single engine failure is 1 in a million, then if I remember correctly from doing probability at school, the chance that both engines will fail at the same time (for purely mechanical reasons) is 1 in a million multiplied by 1 in a million, which is 1 in 100,000 billion.
Generally, multiple engine failures occur due to fuel starvation / contamination or flight into volcanic ash. In both of those cases, a 4-engined jet will not help you, all 4 engines will fail.
I don't think my argument is flawed, do you?
NorthernSky highlights some other interesting safety enhancements with twins which I had never previously considered.
One thing is for sure though, I would much rather fly across the Atlantic in a nice new A330 or 777 than the likes of a DC-10 or 747 classic.