Congrats on 4 years!! Sounds like it has been tough on you, though, so I hope you are able to wean your little man soon and with ease.
I am a total NUB as extended breastfeeding, so I will fumble around with trying to help. Have you tried asking him what it is about nursing that he likes? If you can figure out what exactly he is needing from it, you might be able to find other ways to provide that to him. If he loves the taste of the milk, maybe you can substitute some chocolate milk or a milkshake or something when he wants to nurse at night. Or if it is the closeness and being held, maybe you can hold him and have a special song just for him, or something like that? Maybe he can come up with some ideas of his own on what will help him?
Please let us know how your efforts go so I can learn from it!