"Well it's easy to show mathematically that the probability of at least one engine failing is greater when you have 4-engines vs. 2-engines. But it's not double."
But there is, mathematically at least, a significantly lower probability of a return with only a single engine operating with a 4-engine jet.
Yes, it is, even if one assumes engine failures are "coupled" it probably means that all will fail instead of 3. Either engines never fail together, meaning likely 1 will fail. Or they do fail together meaning 4 will fail. That 2 or 3 would fail seems highly unlikely. Especially 3? 2 is possible if, say, some part that serves one side of the airplane breaks.
Also, external forces of course means more than one engine might fail. Such as ash or birds or (multiple) missiles.