My 4 year old son is misbehaving?

My 4 year old son all of a sudden changed his behavior. He used to be the nicest boy, sweet, helpful, and very smart. People who met him would always compliment me on him. All of a sudden just few days ago this changed. WHen i want him to get dressed, it's a battle. When we have school (homeschool) he acts silly. He just acted silly during his soccer practice yesterday - jumping around, spitting and pretending that he is shooting (He is hardly ever watching any TV so he is not exposed to any violence) I was almost embarred. He started to do bad things to his 2 year old sister on purpose. I am clueless. There have been no changes in our family or house. It's just him who changed all of a sudden. Does this have anything to do with his age? Is this only some kind of stage he is going through and all i can do is just stay on top of it? How do I do that? He went through different stages before. Please help me how to deal with this the right way. Thank you. And he turned 4 at the end of December.