It wouldn't be an ETOPS related problem with Virgin, "4 engines for long haul", now changing.
It could have been anything that would have grounded the A/C in JFK, easier to fix at base.
May have been an engine shut down for something like oil loss or vibration.
An engine shut down on a 4 engine aircraft is not a reportable occurrance under the UK MOR system if it doesn't happen during a critical phase of flight, ie. take off or landing.
At least that's the way it used to be.