Zabbix agents from previous Zabbix versions are compatible with Zabbix 3.4. However, you may need to review the configuration of older agents as some parameters have changed, for example, parameters related to logging for versions before 3.0.
To take full advantage of new and improved items, improved performance and reduced memory usage, use the latest 3.4 agent.
Both Zabbix 3.4 proxies and Zabbix 3.4 server are supported to work only with Zabbix 3.4 server and Zabbix 3.4 proxies respectively.
XML files, exported with 1.8, 2.0, 2.2, 2.4, 3.0 and 3.2 are supported for import in Zabbix 3.4.