I've got both the af3 and the nvb4 and would say your probably better off looking elsewhere, the nvb4 are just OK, and not much if any of an improvement on the 3's.
The Af3 are a weird one, they do give you good energy return, but I still can't figure out where the footstrike should be, and I'm not convinced they are the fastest for a man of my size(13st ish)
If you want to go quick, the saucony endorphin elite are rapid, the Asics metaspeed sky+ are also speedy, hard to tell between the two tbh.
I've just been looking at the endorphin pro 4 and they get great reviews so I'm tempted as well.
Asics are releasing the new metaspeed paris at the start of march but they seem to be only available in red so its a no from me, even though they will be a rapid shoe