NO!Thank you for this tip.
Can I mix Proxmox 4.x and 5.1 hosts in one cluster?
Where in the wiki article does it say that? You can mix 4.x with 5.x for the purpose of the upgrade. On migration of your VMs/CTs you might not be able to migrate back to the 4.x hosts.So, If I want to upgrade Proxmox 4.4 cluster to 5.1: I must shutdown all cluster's hosts and
upgrade host one by one according to this document: https://pve.proxmox.com/wiki/Upgrade_from_4.x_to_5.0
Yes. I think you can use ansible for that.I've found this thread: https://forum.proxmox.com/threads/4-4-and-5-x-version-in-the-same-cluster.37170/