First, the package that will define that the Zabbix repository should be installed:

# rpm -ivh http://repo.zabbix.com/zabbix/4.0/rhel/7/x86_64/zabbix-release-4.0-1.el7.noarch.rpm

Once the repository has been set up, you may install the packages:

# yum install zabbix-server-mysql zabbix-web-mysql zabbix-agent