I too battled with this for the best part of a day before finding the following solution:
Make sure you have a computer name set up in System Prefs > Sharing.
Also, make sure it's less than 20 characters long with no punctuation (dashes are okay). Then click the Edit button below the computer name and make sure the Local Hostname is the same (it'll be lower case here and appended with .local).
If no computer name is set, or if it's too long, Suitcase may well not launch.
In my case, I'd been trying Fusion 4 in demo mode without issue, but after I bought a serial number and activated it, it would no longer launch, so I'm guessing the serial number activation interacts with the computer name somehow.
So if you can't get Suitcase to launch for no apparent reason, and have tried the various solutions documented on the Extensis help pages, this might be worth a look.
Hope it helps.