Screen resolution 4k vs remote desktop resolution (Microsoft Remote Desktop) First, very sorry for my English. I do not write often in English...

I just upgrade from an old iMac 2011 to the new iMac 4k 2017. I am quite pleased with it, but something's bothering me. 

I have to work in remote desktop on another computer, a Dell Optiplex 3040, Windows 10, in another city. The resolution is not very good (probably because of my own 4K resolution). I tried a few tricks - downgrade my screen resolution with SwitchResX, but it just affect my own screen badly. I tried to adjust the resolution inside of the app (Remote Desktop by Microsoft), but no luck.

My question is : if i use an second monitor (not 4k), and display the screen of the MRD on it, could it address the situation ? Could the resolution on the external monitor would look like what I had on my old iMac ?
You can dedicate a secondary display to a remote desktop connection by setting Resolution to Native in the Edit window for a saved connection, and selecting Start session in full screen. You might also want to uncheck Use all monitors. If the resolution of the secondary display is suitable for your Windows 10 system, this is likely to work well. When full screen is selected, the session will open on the display containing the primary Microsoft Remote Desktop window.

You might be able to improve the appearance of a remote desktop on your primary display by using scaling (there is a Scaling item in the Window menu). The Scale content checkbox in the Edit allows you to change the default value for that. You might find that text is too small if you shrink the window too much, depending on how the Windows 10 system is configured.