[Xixi]

On point 4: four engines have some disadvantages, but in terms of fuel consumption the difference is within 1% or 2% of two engines. The issue has more to do with maintenance, and reliability: with two extra engines, you are that much more likely to have one going tech and be grounded.

EDIT: interesting link, that contradicts some of what I say above: https://leehamnews.com/2015/12/11/bjorns-corner-twins-or-qua...

[FabHK]

Also, aircraft are certified to be able to complete take-off with one engine failure.

Thus, twin engines have to have basically 100% reserve, while the 4 engines only need to have 33% reserve (to make up for one engine failing).

[youngtaff]

Norwegian currently have a 787 stuck in Iran with a broken engine, so I'm not sure they can take off with only one engine

[FabHK]

From what I gather, certification requires that they can take off successfully if an engine fails during the take-off run (after V1). I doubt that they're allowed to leave with half the engines inoperational already.

[youngtaff]

With you… like this Thomson plane that had a failure due to bird ingestion during take off at Manchester - https://www.youtube.com/watch?v=9KhZwsYtNDE