I don't have CS4 on my computer, but with Win 10 and CS6 the drop down seems to work. When did you start seeing this behavior?
I Have Photoshop CS4 on my computer still and all is working well within the New File window, Although I am using a iMac.
Have you tried re installing the program?
Several weeks ago.
I am wondering if some adjustment within Windows 10 settings preferences may have influenced this...
It may have something to do with Windows 10 preference settings, so its behaviour on an iMac may not be all that relevant, if that is the cause, but thank you for your reply. I am planning to follow your advice to reinstall, and let's see what happens thereafter!
Reinstalled - problem continues :-(
Did you recently update your Windows 10?
Unfortunately, there isn't much as far as documentation or support since CS4 hasn't been supported by either company for years.