Until now it is not possible to migrate Microsoft Exchange 2016 environments from Windows Server 2012 R2 to Windows Server 2016 by implementing a new Windows Server 2016 with Exchange 2016 CU4 into an existing DAG. This was the recommended way to migrate Exchange Server 2013 to Exchange Server 2016, both on Windows Server 2012 R2.
If you try to implement an Exchange Server 2016 CU4 on Windows Server 2016 into an existing Exchange 2016/2013 DAG environment on Windows Server 2012 R2, you will get an error that this is still not possible because of different operating system version numbers of the involved Exchange Server DAG members. The result is actually that you have to build up a new DAG with Exchange 2016 CU4 Servers on Windows Server 2016. Afterwards you have to move all mailboxes with local move requests to the new databases on the new Exchange Server DAG.
We will have a further look on this issue an may update this post until we will have found new informations!