Did you speak to your dentist about sedation vs giggle juice (that is what my girls' dentist calls nitrous)? Also, did you ask if any of that would be needed? Both of my daughters have had surface cavities taken care of without any numbing at all. When my girls have needed deeper cavities filled we opted for just regular numbing. Neither one of them freaked out or even cried. they chose a flavor for topical numbing, then had the shot. Quick and easy. Their dentist is all about the kids. They give them sunglasses to wear and have monitors mounted over each chair with headphones so the kids can watch movies while they get work/cleanings done. He explains everything to them so they know what will happen doesn't start until they are calm and ready. My kids have never been afraid. Look around for a good pediatric dentist. Visit the office. Get referrals. There is no need for the dentist to be anything but fun for them. I know it is hard, but just walk your son back to the treatment area then cheerfully leave. 'Have fun! I'll be waiting in the waiting area!' Sometimes having M. there looking worried (or even just feeling worried) makes it worse. If you have chosen a good children's dentist, he/she will know how to handle your son's fears.
Now, back to the actual problem. You say he hates to brush so now he has cavities. Kids rarely like to brush, or pick up their toys, or do homework, or whatever. He needs to know there is no choice. There should be no arguing, whining, fighting about it. At four he is perfectly capable of doing what he is told. If he argues about it, there needs to be a consequence for that. For teeth related issues, I would cut out all desserts/sweets for the day if he doesn't brush without complaint. If he keeps complaining he loses sweets for another day.(sweets being not just candy, but anything even slightly resembling dessert, sweetened cereal, jam, chocolate milk, juice, make it really strict to make your point). If he can't take care of his teeth then he can't eat things that will damage them. A day or two without anything sweet will make him re-think his opposition to a simple toothbrush.
At four, he should brush his own teeth then have you go over them to get the places he will miss. Use the pink tablets that show him where he is missing, so he will learn, but you will still need to help at least once a day for the next couple of years to make sure they are really getting clean.