Some children do go back to diapers after potty training.
A child may go back to wearing diapers after potty training if they are regressing backwards in their potty training or if the child is actually not ready to potty train.
For a child to potty train they must be able to hold their bladder and should ideally keep their diaper dry for an hour or longer.
If the child is peeing every 20 to 30 minutes then they are not ready to potty train and should be in diapers until they start peeing less often.
Some kids are not ready until 2 to 3 years old to potty train and some kids are not ready to potty train until 4 years to almost 5 years old.
A child will potty train when ready and forcing the issue or potty training too early can make the potty training take longer.
Although most 4 year olds are potty trained it's normal for 4 year olds to wear diapers.
Some 4 year olds wear diapers for bed wetting or because they are not potty trained.
Although some kids potty train by age 18 months to 2 years it does sometimes take longer for some kids to potty train and be ready to stop wearing diapers.
I've known some kids who didn't potty train until age 5.
Although that is rare but it does happen.
It's perfectly normal for some 4 year olds to not be potty trained and be wearing diapers.
However you should be working on the potty training as the kid would have to be potty trained to be enrolled in Kindergarten.
That is unless the child is incontinent or has other health issues that require the child to wear diapers.