When will your 4 yr old be turning 5??? If it is sometime soon they may allow you to bump him/her to the next age group.
My only complaint about the whole thing is: We have a 5 & 3 yr old sons and I thought ok, no problem if they split them up into different groups. Because I thought they stayed in the club together, just different areas.
NOT SO, alot of the times we had to check my older one into the lab and not the club (be sure to look at navigators closely) So we'd drop off the little one at the club and then the older one at the lab....Well we learned not to do that because the little own thought the older one was going with us and we were just leaving him there.
So we always made sure we dropped them both off at a time they would be in the club together and then if they had to take the older one to the lab later on, the little one didn't mind.