With reference to the 2 versus 4 argument ...
It is interesting to note that one of the marketing strategies of Boeing when introducing the B767 large twin in the early '80's as a competitor to the L1011/ DC10 involved a perverse logic .. and that was that the most likely source of catastrophic failure on an A/C were the power plants, therefore the fewer engines an aircraft had then the less chance of a catastrophic failure ??
It makes sense .. but it is nontheless a difficult concept. 30 years have passed and it has been proved to be correct ..
I.Duke