No, you don't have to buy RHEL4. I said that CentOS isn't a supported operating system. Zimbra will run on it as CentOS is binary compatible with RHEL, many forums users have CentOS install - I run my own server on CentOS.
Documentation for the product is on the ..erm
Documentation page. You will also need to check the forums and wiki for some additional details necessary to get Zimbra running on CentOS 64 bit.
BTW, if this is a Network Edition install then you will need a supported Operating System. Is it NE or the OSS version.