If you must use RHEL 4.x, go into the Dell BIOS and tell the SATA controller to use ATA mode instead of native AHCI. That will make the drive appear as a normal IDE drive to older OS’s. I have to do the same thing with Dell Precision 690 workstations at times when I must use RHEL 4.6. After you have the OS loaded, you can go back and reset the SATA controller to AHCI.
A better solution is to use a newer distro such as RHEL/CentOS 5.2 if possible.