There should be no reason why you need to re-install, Zimbra is easy to install on CentOS 4 - I'm using and several forum members use it.
Anyway, you hosts file is incorrect and it should read as follows:
(the rest of the info in your hosts file is correct) and in your example above you've got localdomain.com. If you can't telnet to a port after the installation of zimbra then you either have a firewall or selinx blocking it or the service isn't running. Make the change to your hosts file and go through the install again and we'll see if we can get you going. I'm also assuming that you have the correct A & MX DNS records set.
127.0.0.1 localhost.localdomain localhost