What if you try rewarding the kids that are being good when the "toughies" are not behaving appropriately? When you are doing circle time and everyone else is sitting nicely, give them a stamp and tell the ones not sitting nicely/quietly that they can try for a stamp next time b/c they weren't behaving appropriately. Our school district is doing something called PBIS (you can look it up online as it's a national thing). It basically is rewarding positive behavior for all kids, not rewarding just the bad kids when they do something good. Not sure how well it would work for 3 and 4, but the stamp thing I mentioned above is similar in thought process.
When DS was in preschool 2 years ago, his teacher put a stamp on each hand when they entered the classroom. If they had a time out, major misbehavior, etc, a stamp got washed off. This was a HUGE deal to the kids. A stamp rarely got washed off, but when it happened, all the kids told their parents
The first thing I did when I picked him up was ask if he still had his stamps. It was a great way to find out how he behaved. Preschool was just under 3 hours, so it'd be a little different for an all day thing.