Where was this child before he started with you (today) was he in another daycare or home on extended mat leave with mom and sister? Was it the younger sister's first day too?

Do you sense he is just not used to being disciplined and has been allowed to act this way without consequence or do you suspect he is acting out because he is unhappy about having to go to daycare?

I would plan some really good snacks tomorrow (like candy and cake) and give him a warning before that if he doesn't behave he doesn't get any. Then give him rice cakes for snack as everyone else eats candy and cake. While I totally don't usually approve of using food as punishment/rewards you need to use a form of punishment that means something to him to get the point across and this might be a good lead in. Repeat the next day with something equally as good in hopes he behaves...then hopefully you have him on a good roll and can work your way back to normal snacks!

It sounds like time outs and lectures won't be enough. Try tv time for the other kids with him in another room with just books. A 4yr old may feel that consequence hit home more than a lecture!