Had it been only the data, it could had worked well ...
Unfortunately the RHEL4 system, having been configured on the server 1 would be having all the device drivers compatible to this HP server's H/w configurations.
It would still work only when you do a laborious job of replacing all such drivers and add a few extra manually. What's the point then?
The disk copy images are usually kept for the following scenarios:
1) When you fear of any crash and need to backup/preserve a well configured system.
2) To facilitates factry default in appliances, when users mess up thing in their appliance/systems settings.
3) To enable you to apply a similar setting across a number of similar machines. Eg. big corporates where IT department keep such a image to install over employees desktops.
Better Install a fresh RHEL4 on the SUN system and then mount the disk mage of this HP system as an additional storage and/or copy the data only -however, ofcourse take the dd snap of that HP machine.
Cheers!!!!!