Just looking at the Release notes now, it will be a fresh install either way, as you can't go from 4.5 to 9.x anyway. I'd guess you would need to do a triple upgrade to get from 4.5 upwards if you didn't want to do a clean run, as there were DB changes, OS changes, etc along the way, so clean install will be far easier, but depending on the size of the install maybe not easy (ie if you have a lot of handsets on there, or SIP devices with manual passwords specified).
Just another thought, as the release is 10 years old (4.5 was released on September 24 2007), you will also need to have a part applied to the app record to be able to convert from Teleworker to MBG, as otherwise you wouldn't be able to upgrade as well.
