This is probably wishful thinking, but is there any chance that once you do your system rebuild to upgrade from 4.X to 5.X, if the IP address is the same on the new server that the CAD software will auto update itself to the new version (assuming you've already made the username valid in the UCM 6.X and associated with the new RMCM Application user)?
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.