Your 4 year old is likely just very comfortable in his diapers.
All his life your 4 year old has worn diapers and have gotten used to them and now he is expected to use the toilet and wear underwear.
I personally believe that it should be the child's choice as to when they stop wearing diapers and go to the toilet.
You can however keep him in diapers and still take him to the toilet to get him used to the toilet but if he still refuses the toilet it's best to back off and wait awhile longer and try again.
A 4 year old boy not being potty trained and who will not potty train is pretty normal.
Although some kids potty train by 3 years old it's normal and okay for a 4 year old to not potty train.
I've known some kids who never potty trained until they turn 5 years old and it seemed they would never use the toilet.
Even my now 10 year old son didn't potty train until he was 6 years old.
I finally gave up after trying to potty train him when he was 4 and just left him in diapers and said that he could wear diapers until he was ready.
Then one day a few months after his 6th birthday he decided he wanted to use the toilet.
So he did it on his own and I was not gonna fight him in the battle that I could not win.
Most kids will however potty train before then if not then don't worry about it.
Your son will potty train when he's ready and not before.
We have to remember that we've been diapering our kids since birth and all they know is to go in that comfortable diaper which is normal to them.
There will come a point though that a child will decide that he doesn't want to be diapered anymore most likely and will go to the toilet.
If not by 6 then I certainly would push the issue of potty training but until then rest easy and let him wear diapers.