4 year olds can still wear diapers although the majority of 4 year olds are potty trained and no longer wear diapers in the day.
However some 4 year olds may still need diapers when on long car trips or when asleep due to bed wetting which is normal.
Even older kids wear diapers for bed wetting or even wear pull ups.
If the kid is nearing Kindergarten age though and they are not potty trained then they will need to be potty trained before they can attend Kindergarten.
Most kids are potty trained by age 2 to 3 years old or at least by their 4th birthday but some kids take longer than others.
Kindergartener students do not wear diapers unless they have a health issue such as autism, incontinence or are disabled.
Normal and healthy Kindergarten students are potty trained and must be potty trained in order to attend Kindergarten.
However most Kindergarten students will wet their pants on occasion which is normal and so it's best to send a change of clothes and underwear to school with your Kindergartener.
A child cannot go to school if they are not potty trained unless they are disabled or have other health issues that make it impossible for the child to not potty trained.
If the child has no health issues and is healthy and able to toilet train then the child must be potty trained and out of diapers before they can attend school.
If a child is not potty trained by kindergarten then the child will not be able to attend kindergarten.
However if the child does have medical problems and they are at least trying to potty train then they would be allowed in Kindergarten.
But if the child has no medical problems and is not trying to potty train then the child will not be able to be enrolled in or attend Kindergarten.
Kindergarten teachers do not change diapers and it can be embarrassing for a child to wear diapers to school.
If your child is nearing Kindergarten age and they are not potty trained then you need to push the issue and get the child potty trained and out of diapers.
Or you will have to home school the child if they are still wearing diapers and are not potty trained.
Kindergarten children normally do have wetting accidents which is okay and teachers deal with that but they must be able to use the toilet on their own.