I really don't think you should be worried at all. Like the others have said, he's just a little boy, he doesn't understand all the "adult" stuff that you might be worried about. All he knows is that's how you show someone you care about them. I have a 4 year old boy also, and he's always kissing his little brothers. (when they're getting along! lol) I'm also having to teach him that kissing on the lips is for family only. He gets confused and doesn't really understand why at times, but he'll get there, as will your boy. All my boy really knows right now is family, (he's not in daycare or anything and I don't really know any other kids) so it's a little hard. But trust me, as for right now, you have nothing to worry about...he's only 4! :P If you really don't like him kissing other kids at all, just teach him that hugs are okay.
Hope this helps and good luck! :D