If you want to run anything before 3.18 on CentOS 5 then you can use the php 4.48 RPM’s here:
http://www.labcoding.com/linux/php-448-and-eaccelerator-0952-srpms-for-centos-5/
Not tried it though! :unsure:
Also, Squiz have their own PHP4 RPMs for RHEL5/CentOS5 rebuilt from Red Hat sources available for our clients. If you are a Squiz Client, our sysadmin team is happy to configure PHP4 on RHEL5/CentOS5 for you.