It's pretty common for 4 year olds to wear diapers and not be potty trained but it's not as common as it is for 3 year olds and 2 year olds to wear diapers.
Some 4 year olds are not potty trained and then a few months later get concept of potty training and decide to use the toilet.
However most 4 year olds are potty trained but it's okay if they are not.
Also some 4 year olds may need to wear diapers for health issues such as bed wetting, incontinence, bowel issues, autism etc.
It is very common and normal for 3 year olds to wear diapers.
A lot of 3 year olds wear diapers and are not potty trained and even pediatricians say not to worry about the child being in diapers during the day until they are at least 4 years old.
It's also very common for 2 year olds to wear diapers.
Although some 2 year olds can be potty trained some 2 year olds are not ready to be potty trained.
In reality kids don't really need potty training as they will eventually get it when they are ready.
If a child is ready to potty train they will potty train within a few days to a week at most.
Even if you were to start the process of potty training a child by 2 years old they may not be completely potty trained until they are 3 years old to 3 years and 6 months.
So in reality if you waited until the child was at least 3 years old or a bit older then the child would potty train much easier.
A child should wear diapers as long as they need too and there's no set age that a child has to be potty trained by.
Some kids potty train earlier and some kids potty train later.
It's much easier and better to just keep the child in diapers until at least 3 years old or even 4 years old and then change their diaper and not worry about them peeing their pants and having to clean that up.