Again what is the version of the ACR plug-in your CS4 is reporting as being installed?
Sometimes an update of ACR does not get properly installed, so it is best to check. If it is not ACR 5.5 or 5.6, you may need to manually install ACR 5.6 manually as described on the ACR download page.
Have you tried the DNG converter? Even the first Photoshop CS can open the DNG format when updated to the last ACR 2.4 for that product. So CS4 can handle DNG files with any version of the ACR plug-in for that product.