[ovirt-users] 3.6 to 4.0 Functionality Elevation

Greetings OVirt Community I have a production system that started life as a OV 3.6 system, and I've recently upgraded the engine to 4.0. I see in the cluster config, a configuration element that indicates the cluster functionality level. It has a drop down that allows me to toggle between 3.6, and 4.0. If I select 4.0 and save, what potential problems could I expect ? Setup; 1 Dedicated engine CentOS7 OV version 4 6 hosts running CentOS7 and OV version 4.0 and 3.6 repos. Main data domain is iscsi over IB. ISO and all other data domains are NFS over IB