The age that kids wear size 4 diapers is between the ages of 18 months to 2 to 3 years of age depending on the size and weight of the kid.
Some kids are larger than others and may need a different size even though they are the same age.
Diapers go by weight for sizing and not age as kids are different weights and are built differently.
But most often a 2 year old would need to wear a size 4 or size 5 diaper and a 3 year old would need a size 5 or size 6 diaper.
A 4 year old may fit into a size 5 diaper or they may also need a size 6 or even size 7 diaper depending on their build.
Most often though size 6 diapers fits kids or little ones until they're potty trained between the ages of 2 and 3 years of age.
As a general rule of thumb, babies tend to stay in size 3 diapers for the longest period of time which is about six months by some estimates.
Assuming six or seven diaper changes per day, this could mean you might need anywhere between 1,000 to 12,000 size-3 diapers before moving on to size 4 diapers.
If the diaper fits too loose then the kid needs a smaller size diaper and if the diaper is leaving red marks around the legs or waist then they need a larger size diaper.