Currently at 560 knots ground speed @ 37k. I'm guessing that means all four engines are operating?
Not necessarily - a 747 can happily cruise at 37k Mach .84 on 3 engines providing it's not too heavy (been there, done that during 747-8 flight test). I assume an A380 would be similarly capable.
Doing a high power run up of the replaced engine is more meaningful.