The primary display at login time determines the windows system dpi, so will have apps scaled correctly and if they are dpi aware they will appear sharp. If app

The primary display at login time determines the windows system dpi, so will have apps scaled correctly and if they are dpi aware they will appear sharp. If apps appear wrongly scaled on the primary display at login, they implemented dpi awareness in the wrong way. So, if you make sure the external 4k is primary before you login to windows, you should have a better experience. On other screens with different dpi basically the only apps that look right are the ones using the latest dpi aware api’s.