Thunderball 2 is correct. I forgot to mention that to satisfy engine out criteria the Classic 747s had to have 4 engines as is the same with the 747-400. I have seen the concept models of the 747-400 with 2 engines and you end up with gear almost twice as long as the 777 because of the engine DIA requirements. And thats a lot of what it comes down to now that we have the reliability that we have. Gear is very heavy, sometimes it is cheaper to have 4 engines and shorter gear, but that is only when the engine tear down cost is less than gear maintenance metrics.
The reason the 747-400 and the 747 Advanced(if it ever gets off paper) will have 4 engines is because the cost of designing a new wing to be efficient with 2 engines, plus the gear requirement is way more than the B will be willing to spend.
Plus I think a lot of 747 guys cruise well knowing there is 4 out there instead of two. They already proved what they thought was impossible by having multiple instances of dual flameout.
