The first question would be why do you want RHEL4? It has been END OF LIFE (EOL) for quite some time and RHEL5 will be soon. RHEL7 is the latest and RHEL6 will probably still be around for a while.

Are you saying you have ISO image files that you need to burn to CD?

From the mkisofs man page (RHEL5 - I have no RHEL4 - you probably want to check your man page)

"Note that mkisofs is not designed to communicate with the writer directly. Most writers have proprietary command sets which vary from one manufacturer to another, and you need a specialized tool to actually burn the disk.

The cdrecord utility is a utility capable of burning an actual disc. The latest version of cdrecord is available from ftp://ftp.berlios.de/pub/cdrecord"

Assuming the ISO images you have are copies of the original CDs then the first one should be bootable and usually you have to tell the burning utility that you want to make the CD bootable when you burn it.