No 4 years old is not too old to wear diapers.
Nobody really is too old to wear diapers and diapers are not just for babies although some people tell kids that.
You should never ever tell a child that diapers are just for babies because that's not true and if the kid needs to wear diapers or the teenager or adult needs to wear diapers then it's okay for them to do so.
Most kids are potty trained by age 3 to 4 years old and sometimes as early as age 18 months but all kids are different and it's okay for the 3 or 4 year old to wear diapers either day or night.
Actually having the child in diapers is much better especially when out in public when the kid needs to pee or poop and there's not a restroom available.
The kid can just use the diaper instead of having to hold it or wet or poop their pants.
Diapers are much more healthy than wearing wet or poopy pants.
It's easier to clean a child up that is wearing a diaper than to have to change pants, underwear etc.
3 years old is not too old to wear diapers.
Although some children are potty trained and out of diapers during the day by age 3 it's not a bad thing if your 3 year old is still not potty trained and still wearing diapers.
There's no set age that a child has to be potty trained at.
Actually potty training a child too early when they are not actually ready can cause health issues so it's best to allow the child to decide when to potty train.
Eventually the child will not want to wear the diaper anymore and will learn to use the toilet on their own.
Diapers are for all ages even if you're potty trained.
However a child should be using the toilet by age 4 at the latest unless they have autism or other health issues that prevent potty training.