Most 4 year old boys are potty trained and out of diapers at least during the day.
But your 4 year old boy is not the only 4 year old wearing diapers full time and not potty trained.
Some kids are not ready to potty train at 4 years old and I've known some kids who didn't potty train until they turned 5 years old.
Yes they were normal and had no mental health issues or other health issues but they just refused to potty train until they were 5 years old.
I used to work with kids in a daycare center and had some 5 and 7 year olds not potty trained but eventually they learned that going potty in the toilet is much better than having to have their diaper changed.
Yes I'm serious about that 7 year old kid who we were watching in our daycare center and we had watched him since he was a baby.
His parents said they had tried before but he resisted using the toilet and insisted on wearing diapers instead so his parents just let him choose to wear diapers.
But we told the parents that if he wasn't potty trained and out of diapers during the day by 8 years old that we would have to terminate him because he was getting too old for us to have to change his diapers.
So he was then potty trained at around 7 and 1/2.
That's a rare case though but most are potty trained well before 5 years old and some kids are potty trained as early as 18 months and some at 3 years old.
If the 4 year old is not having any interest in the potty when they are getting near 5 years old then you really need to just put them in underwear and quit putting diapers on them.
They may fight you on it and throw a fit but eventually he will decide to wear underwear and decide that going in the potty is much better than peeing in diapers and pooping in diapers.
Also explain to him that he won't need his diaper changed anymore.
That may work if he hates having his diaper changed as some kids do.