A TriStar was designed to operate on 3 engines, a 777 on 2. But a 747 was designed to operate on all 4 - and losing a substantial part of your normal level of redundancy is not the same thing as operating a fully serviceable a/c specifically designed to require fewer engines.
The in-flight re-plan for fuel reasons approaching NCE is an entirely different matter. Lots of en-route diversion alternates available and perfectly reasonable.
Having lost an engine in a 4-jet at Vr several years ago at MTOW, even with it safely secured and no further drama, there was no way I was going to press on to destination. Dumped to MLW, landed, then transferred to a spare a/c. Proceding to JFK would, to me, have seemed a much better option than crossing the the pond with your systems redundancy already used up.
I'd be intrigued to know why they then elected to carry on instead of landing at JFK - and whether 'commercial pressure' had any bearing on the matter.
Is it the same in other airlines?