I have Portal 10.3 with federated server 10.3.
I also have a Web App developed with Web App builder over 10.3.
To solve some problems I want to upgrade Server only to 10.4, leaving the Portal with 10.3.
Is it possible?
"Only ArcGIS Server sites using version 10.2 or later can be federated with a portal. To federate, the installed versions of ArcGIS Server and Portal for ArcGIS must be identical. You cannot federate if the software versions don't match. If you've already federated and you're upgrading to a later version, ArcGIS Server and Portal for ArcGIS must be upgraded to the same version."