Sounds great! Can you go on a special outing together? Lunch or morning tea at a cafe? A tea party with his fave toys or a picnic lunch in the backyard? A new park he hasn't been to before? Can you bake or decorate a cake/muffins/biscuits etc together? Watch a birthday themed DVD together? A special art or craft activity (tell him you've been waiting for him to be four so he was old enough to do it!)?