It sounds as if you have a budding lawyer in your home! He has put you into a negotiating position already. Four-year-olds are wonderful!
Maybe you want to re-think the whole dessert thing. (By the way, I LOVE desserts.) When the dessert has become the most important part of the meal, there's a problem. Would it be feasible to drop desserts entirely for a while? If you could do that, then you could concentrate on the most nourishing part of the meal without negotiation/bribery/blackmail/whatever. Once the concepts of meals and sweets are firmly separated, you might be able to re-introduce sweets at the end of lunch and dinner to some extent at a later date.
But if you decide to try this, don't let your little lawyer lead you into an argument or another negotiation (or even a big conversation) about it; just let him know in a firm but friendly way that it's something you and dad have decided. You could say you've concluded it would be better for all of you, and that's that.
You're dealing with two things here. One is, "What is a good meal about?" The other is, "Who is running things?" With a four-year-old, you know this already, of course. Keep your smile and your sense of humor, but be in charge.