You setup looks fine to me. As long as the disk order is not changed between sda and sdd your two systems should work satisfactory.
It is quite common for an installer to fail to implement the boot loader and many a time I have to boot the system without any configuration file.
In order to boot a Linux you need to know its kernel and initrd names which are always stored in /boot directory. This information is accessible as long as you have a Live CD or the installlation CD, which has a rescue mode as it is a Red Hat.
My guess is you got a possibly defective /boot/grub/menu.lst created by RHEL's installer. So using a Grub floppy is the best way to kick start it as you can release the booting instruction one line at a time, as per /boot/grub/menu.lst. Grub will respond to each line and so you can see and sort out the problem pretty quickly. 90% of the problem is to do with the partition references.
You can make the Grub floppy with the current system now.
Just boot up with the installation CD and into the rescue mode so that you have kernel up and running. Ask Linux to tell you where is the directory with Grub's stage1 by
find / -name stage1
You should find the Grub's two essential files stage1 and stage2 are stored in directory like /usr/share/grub/i386-pc-RedHat. Just change directory to it, drop in a floppy and "dd" these two files across
dd if=stage1 of=/dev/fd0 bs=512 count=1
dd if=stage2 of=/dev/fd0 bs=512 seek=1
Once you got the Grub floppy made you are half way there. Just follow "Just booting tips"
If you run into trouble list the content of (the "l" here is a small "L")
here as it tell us the partitioning scheme of your 4 Sata disks and enable us to guess the root partition of your RHEL. If you have let RHEL a free hand to partition the disk it should have installed it in multiple partitions and possibly using LVM too.
Your disk order should be defined by /boot/grub/device.map
List the content of /boot/grub/menu.lst if you can't make it boot both systems.
Finally when RHEL is booted it will load the partitions according to the instruction in /etc/fstab so this file is usefull for others to diagnose your booting problem.