As Branson put it .... 'QUADS do it better'
Just remember with any aircraft the greater the number of engines you have the more likely you are to have an engine failure... 4 engines.. four times as likely to have an engine failure... more true with piston engines however because of the reliability of a modern Jet engine compared to a pistion but the law of probability still apply.
The only thing you really have to consider is the amount of engines which require overhauls and the greater amounts of fuel you will burn based on ETOPS considerations......... What will be more expensive in the long run?