Most 4 year olds are potty trained and don't wear diapers during the day.
However some children are still wearing diapers at 4 years old because they are not potty trained yet and I've known a kid in daycare that I used to work at that was 7 and was still wearing diapers.
Finally he decided that he was getting embarrassed at us changing his diapers and we kept him in just his diapers and in the toddler room so that we could check to see when he was wet or poopy easier.
He had to do baby stuff which he hated and decided that he wanted out of diapers and to go do what the big kids did.
So he finally stopped wearing the diapers on his own and wet to the toilet.
Most 4 year olds we had in daycare were potty trained but there were a few that needed diapers all the time and finally potty trained once they reached near 4 and a half.
There are however thousands of 4 year olds in the world that wear diapers full time though so if you have a 4 year old in diapers they are not alone.