The product that CentOS 4 is based upon, Red Hat Enterprise Linux 4, is being End Of Lifed (EOL'ed). Red Hat will no longer be providing any sort of bug fixes* or security updates* to RHEL 4 which means that CentOS 4 will no longer receive any sort of updates which is (or at least should be) a problem to anyone doing anything remotely serious on a web accessible server.
To make an analogy this would be equivalent to Microsoft ending support for Windows Server 2003. Any server running Server 2k3 would stop receiving updates and it would only be a matter of time before you ran into security problems. Beyond that, if the hosting company ran into a problem they could not solve, they would no longer be able to escalate the issue to Microsoft which is not an attractive situation for any web hosting company. Further still, most hardware vendors would no longer provide support for an EOL'd operating system which is just as bad.
The thing that is a bummer is that RHEL/CentOS is not designed to be able to do an in place upgrade. You can't just run a command and update to the next major version of the software (in this case CentOS 5). You have to perform a migration which, as far as I know, is not something LiquidWeb (or any other hosting company) would help you with as your data and code is your responsibility. Beyond that, RHEL/CentOS changed quite a bit between version 4 and version 5. Namely, PHP went from version 4 to version 5 and IS NOT backwards compatible (i.e. PHP 4 code won't necessarily run on PHP 5). Getting your code running on RHEL/CentOS 5 is something you need to test before performing your migration.
If you (or anyone else) had any questions about this hit me up and I'll see if I can set you straight.
*Astute readers might note that Redhat offers an extended life cycle. It is important to note that it is up to Redhat's discretion to determine what is or is not a critical security vulnerability. It has been my experience that the extended life cycle (i.e. with RHEL 3) was not sufficiently updated to be able to reliably pass a PCI DSS scan.... food for thought.