Just a quick comment on the statement that 2 engines are as likely to fail as 4 and the various advantages etc.
I would say there are numerous considerations:
Twin engines provide better economy for a few reasons, main one is less drag i.e. two less pylons, nacelles etc. Therefore less fuel costs. It Can provide some service cost reduction as well.
Some people think the ETOPS stats on expected engine failure rate are a little skimpy, when, for example, engine failure that happen during take-off e.g. the EK 777-300's RR Trent that shed a blade the other day, are not taken into the ETOPS engine failure stats because it did not happen within an ETOPS segment of the flight plan....hmmm.
Going to a non-ETOPS twin, boeing at one point apparantly listed in-flight engine shut-downs on 737's as 'sub-idle conditions...'
I guess you could also look at uncontained engine failures, where it can be pot-luck where the debris ends-up....take-off in a twin, get an uncontained engine failure that takes out the other engine....bad news. (Never happened I believe).
Assuming an engine failure on a twin, the amount of damage to that engine could affect its drag, e.g. if it can't wind-mill...also nasty if stuck near the upper limit of the ETOPS requirement from a suitable runway. this could possibly cause problems in the viability of reaching that runway, you must remember that, for example, a 777 on one engine will drop to about 10,000', while having the rudder deflected to avoid unwanted yaw caused by asymettrical thrust, makes you wonder somewhat with all that drag the fuel requirements.
Some aircraft need 4 simply due to power requirements as stated already.
I guess in the end money rules, so if you can get away with a twin, it is the way to go. People could argue for a long time about which is better, 2/3/4 engines, no definate answer I would say, as there are so many factors to take into account (not just safety)
Big Discalimer: I only fly sail-planes, and only started very recently, please anyone who would like to contradict, please do, I might learn something!