CentOS 4 Offers Strong RHEL Alternative - page 6
CentOS 4 is based on RHEL 4 sources and offers a viable alternative for small enterprises that don't need enterprise support, services or certifications. From a technology point of view in a month of heavy testing and usage, I found CentOS 4 to be a reliable and stable OS. The community forums and mailing list are great resources and helped me to solve any issues that I came across.
Likely many CentOS users will fall into the category of Fedora users that need better stability but don't want to pay any more than they are paying for Fedora, i.e. Free. While I'm a huge fan of Fedora, it can be a difficult distribution to put in place and maintain for servers (in my case app and file servers) that you really don't want to have to fully upgrade up to three times a year. I suspect that for many technically adept small enterprise users, CentOS 4 will fit the bill as a stable and reliable enterprise Linux offering.