Most 4 year olds are potty trained although some 4 year olds may not be potty trained which is okay.
Also if the 4 year old has autism or other disabilities they may not potty train until near 5 or even 6 and some kids with more severe autism or severe disabilities may not potty train at all.
Although the majority of 4 year olds are potty trained and no longer need diapers in the day but they still may need diapers at night for bed wetting which is normal.
The average age to potty train is between the ages of 18 months 2 years but some kids are not ready to potty train until 3 years old or even 3 years and 6 months.
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.