[ovirt-users] Upgrade datacenter from 3.6 to 4.0

Hi, What is the best way to upgrade a datacenter from ovirt 3.6 to 4.0 ? Upgrade engine, hosts then cluster ? If right, how do you upgrade a host running ovirt 3.6.7 ? Simple upgrade or fresh install ? During the update process of a cluster, some hosts will be in 3.6.7 and others in 4.0.2, is it a problem for migrating VMs ? Found docs about engine upgrade but not a lot about host and cluster. Thanks, Arnaud