4.0 for sure, but within the 4.5 ranks, there are cherrypickers and there are guys who stretch to play at that level.
Lots of former Div1 MEN play at 4.5 levels, 10 years out of school.
Div1 woman would be a strong 5.5, or about a 4.8 level man.
Then there is the question of matchups. Some females can crush returns of serves coming in at 130mph. Other's might get intimidated by a big strong net rusher and server.
It'd be close.