Its not really true that by definition four-engine planes are more fuel inefficient per seat. A A380 with new engines on a per seat bases isn't that inefficient

Its not really true that by definition four-engine planes are more fuel inefficient per seat. A A380 with new engines on a per seat bases isn't that inefficient at all. It is just a question of if you can fill the space efficiently. That is why some of the Hub-and-Spoke Airlines are streaming for a A380NEO > Customers want smaller aircraft point-to-point travel, not larger spoke–hub travel Yes, but in the real world many Hub-And-Spoke Airlines are very successful. And with airport extensions and new builds not going up as fast as travel demand, Hub-And-Spoke isn't going anywhere anytime soon. It will become more important. > if they show the FAA a good quality spreadsheet and perform a few trivial tests. Its not that easy. There is a reason private jets still have 3 engines, getting certified is to expensive.