I am currently standardizing on CentOS 4 at PARI.
In particular for our new Plone site we are rolling out, CentOS 4 really works well, as long as Plone/Zope are installed from source; the available RPMs are very poor, but the from source install is very easy with very few dependencies to track down and virtually everything you need in the Plone tarball (except for Zope itself).
I also find OpenACS runs well on CentOS 4 with the CentOS4-provided PostgreSQL RPMs. I am using that combination on my personal website at www.lamarowen.net, on which I still have lots of work to do.
