I just rebuilt, and did a clean Windows and P3D install.  My hardware setup is the same and I switch between using a regular HD (1920x1080) monitor and a 55" 4K TV.  I never really use them at the same time, as they're not quite next to each other. 

Before, I had some setting selected that allowed them to scale well and I could switch easily using Display Settings->Show Only on Display 1/2.  The desktop would appear effectively the same, i.e. columns of 8 icons on the desktop, windows appearing about the same relative size. 

Now, when I do this the 4k TV appears double the size (i.e. columns of 4 icons, open windows occupy about double the space).  I can set a custom scale to make it look "normal", but I have to reset this each time I switch now.  

For the life of me, I can't find what this setting I had before was that allowed them to automatically scale so the desktop appeared the same. Could anyone enlighten me?  Thank you!

have you tried right clicking and going to display settings then changing 

"Change the size of text, apps, and other items"?

On 1/6/2021 at 6:59 AM, micstatic said:

have you tried right clicking and going to display settings then changing 

"Change the size of text, apps, and other items"?

I know this is a delayed response, but that seemed to be the option I needed.  After your response, I looked at my options again.  Initially the "Change the size of text, apps, and other items", along with resolution were grayed out on my TV.  They showed up on my monitor fine.  After either pressing detect or identify, the options opened up and I was able to set my TV from 300% to 200%, which made it look just right. 

Great