Where do you live, Stacy? You will find that there is a HUGE amount of variability in the US. For one thing, there are parts of the country (like California) where you will have separate women's fields and the CAT4 field will fill up with 50 women (the maximum). There are other parts of the country where all cats (1-4) will be combined in one race and you'll only have 10 women total. My suggestion is to chat with some of your local racers to find out more. You can also google for specific race reports to review races (the writers will typically include pace). Better yet, find a club to race with!
Here in California, a typical W4 crit will average 22-24mph. A road race, depending on how hilly it is, will average between 16 & 22 mph. We don't have many flat road races. For time trials, the winners will be averaging 22+ mph.
Now don't panic about how fast these races are. With the exception of TTs, you will be in a pack, and you can go much faster in a pack than you ever could on your own.
Good luck with racing! It's a great sport. Don't hesitate to ask if you have any other questions. I've been coaching women racers since 2002 and there are also some other great women on this board who have raced and can share their experience with you.
Lorri
http://coaching.velogirls.com