Most kids are potty trained before 4 years old. But maybe your friends son is having trouble with potty training or something else is wrong with them.
If the kid is able to potty train then most likely the 4 year old boy will be potty trained before 5 years old.
Not all kids are the same and some potty train at different ages and wear diapers longer than others.
Some babies are potty trained at 18 months while others take longer and there's nothing wrong with it taking longer for a kid to potty train and get out of diapers.