4.0 average serve is 90mph or recreational level women can easily just block back 95mph serve - can't decide which one is more improbable....
that seems reasonable on the high end... not the average, but reasonable.
for many crossover athletes 4.0 is a transient level, on their way to 4.5... but they are also a 4.0 because they only get like 30% of 1st serves in, and maybe even hit a flat serve as their 2nd serve, which explains the high mph 2nd serves
regarding the 4.5 women i play, they seem to all be able to block back my fastest flat serves, presuming they don't have to move.... but they are terrible at getting back anything with alot of spin/movement (kick/slice)
recalling my stats from using the sony smart sensor:
height: 5'4"
ntrp: low2mid4.5
1st serve avg: 85mph (topslice)
2nd serve avg: 65mph (kicker)
i can hit flat serves, mid90's, topping out around 97-98, but a few years ago, those serves rarely went in, so i never used it in a match.
in general i'm a "spot/changeup" server rather than a "fast ball" server, presuming that the serve will come back, but hopefully put me at an advantage at the beginning of a rally. if i ace you, it was an accident, or your were cheating/anticipating the wrong way.