Are twins inherently more efficient per pax than quads? Wouldn't a quad plane just run at a lower thrust level per engine? Given that fan sizes and core temperatures are rammed up against difficult limits, I would've thought that quads could give a better bypass ratio (and hence efficiency) by reducing the core engine size and thrust requirement.

Or do you mean operational efficiency...fewer parts to maintain, engineers, simpler plumbing etc? Probably less disturbance on the wing flow with 2 engines as well.