Follow these steps and hope will resolve the issue -
# rpm -e zabbix-release
Now, install the package you want to install and follow below steps.
# yum info zabbix-release
# yum list zabbix-release
This will clear from your yum cache also.
# yum clean zabbix-release
# yum clean all
# yum install zabbix-agent
There are two methods. It's really statistics. The first method (as by OP) leads to:
1/4 (--every 4) gets first choice
1/3 (--every 3) of remaining gets 2nd choice
There are only two rules in OP's question but one can assume two more (or one more plus default) would follow:
1/2 of remaining gets 3rd choice
remaining gets 4th choice
As iptables' nat table ...