For the upgrade from 2.x to 4.x I will follow the upgrade procedure to get all of the existing info. I follow that. Should I then blow away the Operating System on current Unity server, install the OS and then do a fresh install of 4.x?
That'll work - you can't go wrong with a fresh install. If the Unity 2.x server is part of a larger Exchange site/org, however, be sure to run the uninstall to remove unity objects/properties from the directory before waxing the box entirely. If it's a stand alone system and you're starting with a fresh OS install then there's no need for that.
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.