Some folks are noticing that we added a Masters Cat4 class. That means if you want to race the 10:00 Masters 1-3 race you need to cat up. If you raced a lot you can cat up (10 races or 10 USAC points), or if you have a Cat 3 in road or a Sport/Cat 2 in mountain biking.
Note there is a typo in red at this link. http://www.usacycling.org/
Bonus that masters Cat3 racers can do 4 races a day.