Hi Mark R,
I understand your concern, particularly since increasing Windows scaling further would affect the rest of the interface.
In File Explorer, try Ctrl + mouse wheel up while viewing the image folder, or choose View > Extra large icons. This increases the folder view without changing your system-wide display scaling.
If the maximum thumbnail size is still insufficient on your 4K display, consider an image viewer/file manager that supports custom thumbnail sizing. You can also press Win + F to submit this requirement through Feedback Hub.
Please note that this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas.
Hopefully one of these options makes browsing your images more practical.