leadie, the airline I work for has a large fleet of both 380 and 777. It has found that the 380 does better than the 777 on ULR sectors, as the 777 is MTOW limited when sectors exceed 12hrs and seats/cargo needs to be blocked/left behind. The 380 is carrying MZFW out to 15hrs. On flights from DXB to LAX, for example, the 773ER would have 90 seats blocked, so 265 seats and burn 8.6t/hr. The 380 is carrying 517 and burning 12.7t/hr (15hr sector). I do understand that QF only has the first generation 380 so may have different numbers, as our newer Aircraft have a greater MTOW and a lesser burn than our original ones.
The two beats four argument does work when a 777 is lined up against a 747 as they effectively carry the same load, all be it with the 747 being able to take a greater load further. But when the quad carries significantly more, the twin advantage is reduced. For example our 772LRs burn more per seat than our 380s.
It's all well and good to have a low fuel burn, but to make it work you need to be able to carry an economic load.