riders dropping out of the 4s to help their buddy in the 5s is definitely a no-no as they are not in the same race. that's different than riders on different laps working together (like in a crit as long as someone doesn't purposely drop back to help a breakaway). in the races around my area usually separate the numbers and have different backgrounds to help identify which race a riders in. so one group will have a yellow background and numbers 200-299 whlie another catagory will be 300-399 and have a white background.