A fully supported workaround would be to install a supported OS with virtualization support and run RHEL4 as a virtual guest OS on top of it.
The "Production 2" life-cycle phase of RHEL 4 ended at the end of March 2011. After that point, no new hardware support was going to be added to RHEL4. The end of February 2012 marked the end of the regular life-cycle for RHEL4.
The 8th Generation of Proliant servers was introduced in early 2012, so adding hardware support for Gen8 to RHEL4 was out of the question.
If you have Linux kernel programming skills and time, it might be possible for you to backport the "hpsa" driver to the RHEL 4 kernels on your own, and package it as a Driver Disk to allow installation. But that might be a fairly major task.
