[andyjenn]

It's the 4 engines which gives airlines A380/747 fuel cost problems. Both fulfil relatively niche markets and are gradually being displaced by the more-efficient and flexible twin-engine A350/787 types. We're just starting to see the secondary market for A380s now which is a good indicator for its long term commercial viability.

[dingaling]

Twins are actually overpowered compared to comparably-sized quads due to engine-out climb requirements. In terms of raw seat-km-litre they are therefore slightly less efficient.

A good example is the old A340-300 quad, often maligned for being underpowered. Now that it is mostly retired data has emerged that it was more efficient than the rival 777-200 twin.

Where twins are cheaper is in terms of capital costs and consumables, though ETOPs requirements mean you need more technicians to service a twin ( separate teams for each engine ).