When a child reaches age 4 and has no disability or has no bladder issues etc then they should be getting out of diapers during the day and should be potty trained.
Ideally a child should be introduced to the toilet by age 2 or 3 years old and should be fully potty trained by age 3 or 4 years old.
Some children are ready to stop wearing diapers and be potty trained by age 18 months while others may not be ready until 3 and 4 years old.
But if the child is not potty trained by 4 years old they should be forced into it because they must be potty trained during the day and out of diapers before going to preschool or kindergarten.
If the 4 year old needs to wear diapers at night that's okay but they should be out of diapers during the day at age 4 and certainly before age 5.