More info here...

just the info on the 4s being split:
Men's Cat 4 "A" - Race participants that have an annual USA cycling license as a Cat 4 or 5 that have raced 1 year of more in road or cyclocross. One day license purchasers may be permitted to race 4A by the choice of the individual. The overall winner of the 4A groups will win the 2007 Chicago Cyclocross Cup Jersey and will be eligible for Illinois State Championship medals (only Illinois Residents). Scoring will be tracked for both A's and B's separately. Once you have decided to be a Cat 4A you cannot compete in Cat 4B races for the remiander of the series.

Men's Cat 4 "B" - Race participants that are truly beginners that do not have an annual USA cycling license, or first year license holders (Road as Cat 5). Also participants that competed in Cyclocross 1 year or less. Non-licensed participants must purchase a USA cycling ONE-DAY license for $10 for each race. The winner for the overall is NOT eligible to win the 2007 Chicago Cyclocross Cup Jersey or the Illinois State Championship medals. Cat 4B racers may move to Cat 4A races with the stipulation that no points will be carried from B's to A's for overall points determination.

based on the number of riders in the first race, this is a good idea. There were 79 registered racers at Jackson Park.