Yes, this is an issue.
it’s surprisingly hard to get genuine data on it though, not least because many 4 year olds are really not entirely reliably toilet trained (either now or in the past).
i worked in a primary in the 2000s and every year the reception teacher asked for new parents to send in a spare set of clothes for the children because there were inevitable accidents.
she’d send the class to the toilets in pairs before morning break, before lunch and before afternoon break, and the class had it’s own loos integrated into the reception area (which is standard as many 4 year olds need the loo at short notice).
most of the children in reception had an accident at some point in the year to be honest and some had multiple accidents. But officially they were all toilet trained.
we didn’t have any children in nappies but we did have children with bowel issues - some with allergies, one with crohns - who sometimes needed a whole change.
most 4 year olds are somewhere on the toilet training journey but few are completely reliable. I’d guess that these days there are more who are less reliable as parents tend to start later these days, but most parents are aware that schools expect children to be toilet trained and I’d expect most of the ones that start in nappies to have significant other issues.