It looks like the august edition of the race has a 3/4 field. The july 29th edition has a cat 4 field. When I follow the links on NCNCA for the august race it takes me to the reg page for the july race. Sara's reged for the cat 4s on the 29th.
Normally you register for a field that's appropriate for your age and cat. And then at registration they verify that you're legal for that field. If you sign up for the wrong field then they'll catch it at reg and move you. If there are any spots open (not normally a problem for women).