In my test lab I have installed RHEL3 U8 and RHEL4 U4 on a single hard-disk with '/boot' for each RHEL version. I have attached the screen-shot of how to install. Please follow the screen-shots accordingly.

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the output of 'fdisk -l' and 'df -h' from the RHEL3 U8

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the grub.conf from RHEL3 U8

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RHEL4 U4 installation

You don't have to create swap partition again, only one swap is needed.

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do not install bootloader

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'fdisk -l' after booting to RHEL3 i.e after installing RHEL4. See the additional partitions created.

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the '/boot/grub/grub.conf' file of RHEL3 U8 with entries of RHEL4 U4 added.

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the label of the '/' and '/boot' on RHEL 4 partition. You have to change grub according to this label.

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Note the root (hd0,4) in the grub.conf file. This is the '/boot' partition of RHEL 4. Please note this is one number lower than the actual version.

You can follow the same procedures during installation.

If you face any problem then please attach the sysreport from the system for further analysis.

The steps for collecting sysreport of the system are available at following link: http://kbase.redhat.com/faq/FAQ_80_456.shtm