I think that if there is a fourth race then, I would make it that you only have to race 3 of the 4 races, it could be hard for some to make four races. As you stated, it definitely has to be at a different track and I would not do it on a holiday weekend. I think a holiday weekend can become too diluted with other things going on, but who knows.
Lastly, have any of the tracks considered having a one day pass for riders that don't want to do a whole series (or buy a membership) or it may allow a person to try racing to see if they like it. Make it only good for one day and if they want to come back they have the choice of buying another one day pass or buy a membership. However, if they only want to race 3 times it only cost them $15, they get to race, you get $15 of revenue that you may not have gotten and lastly, maybe it helps fill the gate with an extra rider or two. Also, you get the class fee for whatever "classes" they sign up for. Also, a one day pass does not allow you to come to the banquet. The banquet is for members only.