Can we use older agents with newer Zabbix servers? Can I use a Zabbix 4.0.1 server with a Zabbix 4.0.0 frontend and some 3.0.3 agents? When upgrading from Zabbix 3.4 to 4.0, can I keep my proxies on 3.4 after I've upgraded my Zabbix server to 4.0?