====== Install Zabbix Agent 4.0 on Ubuntu 16.04 (Xenial) LTS / Debian 9 (Stretch) ======
-- //Tested with **Ubuntu 16.04 (Xenial) LTS / Debian 9** and **Zabbix Agent 4.0 beta 1** on **September 2018**// --

==== Add Zabbix 4 Repository =====

First of all, add Zabbix 4 repository (4 is in beta, hence is version still numbered as 3.5). For **Ubuntu 16.04** use 
  echo "deb http://repo.zabbix.com/zabbix/3.5/ubuntu xenial main" | sudo tee /etc/apt/sources.list.d/zabbix4.list > /dev/null
(build are availble for bionic, trusty, and xenial - set the repo accordingly)

For **Debian 9** use
  echo "deb http://repo.zabbix.com/zabbix/3.5/debian stretch main" | sudo tee /etc/apt/sources.list.d/zabbix4.list > /dev/null
(but jessie and wheezy builds are available, too).

Add the missing public key for repository with
  sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 082AB56BA14FE591
 
and update the apt package list with
  sudo apt-get update

==== Install Zabbix 4 Agent =====

Install the Zabbix Agent
  sudo apt-get install zabbix-agent

==== Configure and restart the Agent =====

Edit your local configuration with 
  sudo vim /etc/zabbix/zabbix_agentd.conf.d/mysetup.conf
and set your custom parameters (at least the address of the zabbix server)
  Server=1.2.3.4,zabbix.example.com

Finally restart the agent with
  sudo systemctl restart zabbix-agent