Some 4 year olds still wear diapers because they are not potty trained, they're not ready to be potty trained or they even want to wear diapers because they feel good.
Also some 4 year olds may need to wear diapers for bed wetting or for long car rides or on outings when they may not make it to the toilet.
As a diaper lover I would allow my child to wear diapers and use the diapers as often as they want too.
I would actually likely not potty train my kids to begin with so they would not use the toilet at all.
I would also probably remove the toilet from our house and it would be diapers and no toilet use.
If there's no toilet in the house to use then they would just use diapers.
But if they were potty trained though and wanted to wear diapers again at any age they could do so and I would also buy the diapers for them.
I would also change the kid into a diaper and bathe them if they wanted me too as well.
There's no age at which I would say no.
I wish I could wear diapers full time but can't right now.
Although hopefully in the future I can wear diapers full time which would be great.
Then I would no longer need a toilet as the diaper would be my toilet.