Getting Windows to use a 4k display properly

I've got a 2013 MacBookPro with the the latest version of OS X and I'm trying to run Windows 10 on an IIyama 4k screen.
MacOS is running at full resolution, but Parallels is running Windows at something like 800*600 which rather spoils things. When I dig into it, the Windows display setting app you get by right clicking on the desktop thinks its running at 1900*1200, but what I see seems to be less than that. Sometimes a flag comes up saying the monitor should be running at its native resolution of 800*600.
I've tried all combinations of the Best for Retina/Scaled etc options Parallels offers. Even rebooting each time. No difference.
When I run Windows in a window, when I re-size the window, it just stretches what is in the window rather than increasing the display available to windows. This would be ok, but when you start up it up, it defaults back to about 800*600.
Finally, the driver Windows thinks it is using is a default one from 2006...
Can anyone help? Please?

 

Hello @WillD1
Could you try changing the scale to fit screen option to 'Auto' and check how it works. Also reinstalling Parallels tools, Actions->Reinstall Parallels tools would be suggested.

 

Thanks for getting back Sasti. I'd tried the scale to fit option - no joy. Most of the time, the reinstall option is greyed out. When it's not greyed out, I tried it twice and nothing seemed to happen.

 

I managed to get it to reinstall, and I now have a 4k screen!!! But. When it fires up keeps putting up a dialog box saying it needs to log out to apply the new shared profile settings. Time and again :-(

 

Hello,
Could you try this solution to fix the shared profile notification message.
Note : Though the article says it is applicable to previous version of Parallels Desktop, the solution is the same for latest version.

 

All sorted. Many thanks for excellent support Sasti!
(seems like the installation people could do with a nudge mind you )

 

Hello WillD1,
Glad to hear! Feel free to reach out anytime.
Thanks,
Guna