Higher complexity doesn't generally improve your reliability aspects though and whereas I don't know much about the 747 it's entirely possible the 4 engines aren't entirely independent.

They might share fuel-pumps per-wing, so if say something happened to fuel pump 1 out of 2 you might not have engines 1 & 2 while engines 3 & 4 are calmy doing their thing.

Less moving parts is generally always a good thing, and if it weren't a passenger aircraft potentially eliminating down to a single engine "might" be acceptable if the gliding capabilities were very good (much like some turbo-prop planes) and the risk of losing life was overall lower.