I want to thank everyone for adding these comments - this is unexpected. I did indeed try editing the registry. Unfortunately I found it affected only those apps that were not really a problem. The worst offenders were not affected at all, the narrowest of the scroll bars stayed exactly as they were. I am guessing the problem arose because these programs set the scroll bar width to a certain number of pixels, and on a 4K screen it is just not adequate.
Possibly this will catch someone's attention at Microsoft...