Even for middle school & high school age, I'd say that realistically after 3 hours for normal practices, you lose their focus. Will they spazz out like the younger groups if you keep them longer? No, but what you get accomplished in that extra hour is typically not much in my experience, since the kids are already tired. They just get frustrated and we end up having to work on things that normally would never be a problem,i.e. stunt/pyramid issues, cleaning, sloppiness. If its a camp or choreography session where the kids are being switched to work on different activities throughout the day & there are some snack breaks/teambonding mixed in, then 4 hours can work however.