My DD1 was exactly the same. She just went on and on sucking! And I have also nooded off during a NF only to wake 1.5hr later and find her still sucking
How long after he starts feeding are you trying to stop? If he protests it could be hunger or he could like the sucking. At four months you could perhaps try a little distraction and see if that helps, eg a song or tickle or rhyme or funny face. If it helps, he's probably had enough milk. If not, he may still be hungry.
Iirc, it was at about 4 mos that L's feeds suddenly got a lot shorter. She just becamea lot more interested in her surroundings and doing things, and only wanted to eat, not suck for comfort. Maybe your DS will be like this soon too.