Originally Posted by manuk
The solution would be re-format the USB drive
I agree. That's the solution.
I do not know whether an elegant method exists to get RHEL4 to handle ext4 filesystems. But I'm fairly certain you will be going into unsupported territory with Red Hat. (i.e. They're not going to help if/when you break things.)
Rather than mucking about with a production system in this way, I recommend formatting the USB drive. Simple and low risk.