Current upgrade procedure is not enough documented (https://www.zabbix.com/documentation/4.0/manual/installation/upgrade/packages/rhel_centos). For example, it has no answers on the important questions:

• what difference between "Upgrade procedure" and "Minor upgrade procedure"?
• we have "Minor upgrade procedure" section, but where is the major?
• when should I use major/minor procedure?
• what should I do in in case of "upgrade failed" scenario? If the root cause is fixed, how to continue upgrade (should I restore DB from backup or just restart Zabbix)?
• why should I use different commands ("yum upgrade zabbix-server-mysql zabbix-web-mysql zabbix-agent" VS "yum update zabbix-*")?
• the "test upgrade" step is missing

The upgrade process it is important! It would be great to provide more details about it.