I guess it depends on the area and the race, but from what I've observed the women's races seem completely different than the men's races. Smaller fields, wider spread of abilities (strength and skill), less agressiveness, etc. Combined/open fields are even worse. But that's from an outside looking in perspective, obviously I have no firsthand experience...
Edit: The women that I know that train and ride with men tend to race more like men. More agressive, attacks, etc. The others chit-chat.
Yeah, a lot of the time the field can be much smaller (depends on where in the country). But I think that the range of skills and strengths for a women's cat 4 race isn't that different than most cat 5 races. The only difference in skills might be for the veteran cat 4 women.
I have found that beginner women who don't train with men are less likely to try racing.