Screen resolution 4k vs remote desktop resolution (Microsoft Remote Desktop)
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.