[AceyMan]

The dagger in the heart of the big 4-engine aircraft are the changes in the ETOPS allowances¹ by the regulatory bodies.

Used to be you needed the redundancy of four power plants to provide the needed nines to cross the ocean and still ensure (well, meet the statistic standard) that you could make land with one engine out.

The reliability of the modern turbofans allow the big twins to take almost the same transoceanic flight path as the 4-engine planes. Considering the complexity and cost of running 4 engines vs 2, it's a no-brainer to give up a few percentage points on ETE for a massive cost savings.

(me: licensed aircraft dispatcher)

¹—https://en.wikipedia.org/wiki/ETOPS?wprov=sfla1

[AceyMan]

(I hope that comment wasn't a dupe; I was on mobile and didn't see anyone else yet go over that aspect of the sitch.)