Assuming the engines to be perfectly reliable (100% reliability no matter the conditions),
With such an assumption then we can presume that folks will fly in single engined airliners - why waste all that money on more than one really needs ?
Put another way, of ALL diverts of aircraft flying hours away from the nearest suitable, how many were due to something NOT engine related?
Don't have any figures to hand. However, systems other than engines are the driving concern in recent years - multiple electrical failures, on board fire are but two which spring to mind.
what is the chance of a 4 engine suffering dual-engine failure compared with a twin suffering dual-engine failure?
Somewhat higher, I suggest - two engines are on the same side. If one lets go in a big way, there is a rather higher probability that debris will take out the adjacent engine rather than (the) one on the other side. Plently of examples around to back up that thought.