Mine were four and five and thirteen months apart. We left them in the groups they "belonged" in.
A lot of club time they were able to spend together, but the 5-7s spend some time over in the lab and do some different stuff.
The 3-4 activities seemed pretty babyish - fortunately my daughter doesn't mind being little, but at 4 1/2 she was pretty beyond some of the games they were playing. I watched them do a matching game, and she had it down while most of the other kids were trying to figure out what they were supposed to do.
On the other hand, I really wouldn't have wanted my four year old hanging with the seven year olds. She wasn't really ready for that either.