2) As for dynamics...yeah, I'll agree with you on that. Women can be catty. That doesn't mean we are catty, but we can be. Where does that leave us though? It actually appears that we have to prove ourselves above and beyond. "I won't be catty, I won't go after my co-founder and hopefully he won't come after me, and I'm sorry I'm not a dude so I can't create an all male cohort." Doesn't mean it will stop me but it does make it more difficult.
3) Call me biased because I'm an educator, but I think a big obstacle is our science education in this country. Or lack thereof. Tech information is geared towards males, not females and it is a poor show at that. If more females became involved at a younger age, I think YC would have a hard time keeping the women out. :)
As far as the marriage/kids issue goes ... I think I read that the average age of the people at YCombinator is 26; and as far as my own observation goes, that's right around the time that my female friends started seriously thinking about the marriage/kids issue. I think the discrepancy is really driven by the biological clock. Men can have kids well into their 50's, but it's pretty dangerous for women to have kids past 40.
Maybe I'm too much of an outlier, but to me, achieving my dreams is more important than having a kid, which has never been something I particularly prioritised in life. I have at least 5 more years before I really have to worry anyway. Plus, we're having babies long after 40 now.
1) As jlees mentioned below, you need to look at the "subset of women hackers who also have some entrepreneurial drive" when looking at something like YC. The 4% doesn't really surprise me at all.
2) My observation is that women tend to be on a timeline about 5 years "faster" than men because of their biological clock. As in, they'll start thinking seriously of marriage and kids late 20's, early 30's whereas men will deal with this issue more like early to mid 30's. So their window for doing a startup is smaller.
3) This is a bit of a generalization, but the dynamics are really different when you have all-guy co-founders versus mixed versus all-female co-founders. With mixed sometimes personal relationships develop which can make things messy. And when you have a couple of alpha-females together, sometimes that can also lead to problems (for example, see the all-female startup in the TechStars Reality show). [These are just my own observations. Please don't flame me for this!]