I looked up today (in the Peak District in the north of England, if you care) and saw that two of the nine vapour trails I could see were clearly from four-engined jets. Given that airlines are using a fraction of their fleets, and not all the fleet will be four-engined, why would they be using their least efficient jets? When load factors are so low I’d have thought that’d be the worst thing they could do?

Long story short, which airlines are flying 4E and why?