Yes most 4 year old kids are potty trained and out of diapers at least during the day.
But sometimes the 4 year old child may still need to wear diapers at night because they are wetting the bed which is normal.
But most 4 year olds can both poop and pee in the toilet on their own during the day and do not need to wear diapers anymore.
However there can be some very stubborn 4 year old kids who refuse to potty train and want to keep wearing diapers.
So if the 4 year old is still not wanting to wear underwear and wear diapers instead then there's no point in really pushing the issue.
But if the child is getting near 5 years old and about to attend kindergarten and they are still wearing diapers then you need to stop diapering the 4 year old and make them use the toilet.
Put the child in underwear and let them pee and poop in the underwear and then eventually they will decide that they don't like being wet or dirty.
Or switch the child to cloth diapers if they are in disposables as they will feel wet and soiled in the cloth diapers and may decide to potty train.
Let them know that they are a big kid now and that big kids use the potty and they will no longer need to have their diaper changed everywhere they go.
Using the toilet will be much better for them.
Eventually they will use the toilet on their own though but at 4 years and 6 months I would push the issue.