It's perfectly normal and very normal now for a 4 year old to wear diapers.
Some 4 year olds are not potty trained which is okay and they will potty train when they are ready and not before.
Although a lot of kids are potty trained by age 2 years to 3 years old it's not unusual for a 5 year old and even a 6 and 7 year old to not be potty trained.
My own nephew who is 6 years old is not potty trained and he has not health issues but he just isn't ready.
So he's not potty trained and wears diapers all the time.
His dad said that he's not worried about it and he can afford the diapers.
If he potty trains he does but if not then it's okay.
He wears a size 7 Pampers Cruisers diaper and he seems to like them as well.
The average age for potty training kids is between 18 months to 2 years of age.
That is for most kids who are ready to potty train at those ages.
Some kids are ready to potty train and get out of diapers as early as age 18 months and are fully potty trained by 2 years old.
The child may still need to wear diapers at night for bed wetting but they should be able to go all day without a diaper.
Some kids may not potty train and be out of diapers until age 3 years old to as late as 4 years old.
A kid will eventually potty train when they are ready.
It's normal for a kid to need to wear diapers for bed wetting from age 3 to as late as age 8 years old and even older.
Some kids will continue to wet the bed for quite awhile and it's normal and some kids may wet the bed until age 17 or 18 years old and some people wet the bed all their lives and that is also normal.
Forcing potty training too early can be harmful to the child's health because they may try to hold in their poop or pee and that can lead to kidney and bowel issues.