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As far as I know 4K is not supported with Teams. A request has been posted on the feedback hub https://feedbackportal.microsoft.com/feedback/idea/d412260c-a669-ec11-a819-000d3a058885

If it's a CVAD session, use Screen Sharing: https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/graphics/screen-sharing.html

Hopefully MS will improve MS Teams and will support higher resolutions.