DS 2 (now 18) was like this, except he was 3 instead of 4.
What we did with him was took him to the store to look at ordinary underwear (we called them 'toilet pants', as opposed to diapers or pullups - you couldn't pee or poop in them, they were to wear for when all of you pee and poop goes in the toilet). He was totally into construction machines in those days, and found some toilet pants, I mean underwear, with machines on them. He REALLY WANTED those underwear.
So we made a calendar chart for him, and told him that if he pooped in the toilet for the next 2 weeks then we would make a special shopping trip to go buy him those machine toilet pants. We let him pick out a sticker to put on the chart for each day that he pooped only in the toilet.
Starting the NEXT DAY, he would poop in the toilet, and did that for 13 out of the 14 days. Well in our book that was good enough.
We still remember that shopping trip, it was a very special occasion, he was SO PROUD and felt like such a BIG BOY. So I gave him the money so that he could pay for them himself when it was our turn at the caisse.
So long story short - is there something he wants to have that they can bribe, I mean reward him with? Because it sounds like he's physiologically ready if he knows when he has to go and when he went, it's now mainly a psychological leap.
They might also see if there is something about the toilet that he is afraid of. Two of our boys used a seat that goes on top of the regular seat (we never used a separate 'potty'), the other one preferred to hold onto the regular toilet seat to prevent him from falling in.
Anyway, good luck! He will start going in the toilet at some point, I promise!!