I have it in my garage waiting to go on a 68 charger. That car was previously running a previous pro-flo multi-point EFI system. The difference was it wasn't a self tuning setup. After several costly trips to the dyno to try and get a base set up I saw the light...
The Pro-Flo 4 has a LOT of improvements over the pro-flo 3 from what I can tell. It is a very a nice kit. I wish I could tell you how it runs but I haven't had time to install it yet.
There was a big height difference on the manifolds though, so the new kit required a hood scoop to fit the charger. Also something to note, I blew up 3 or 4 of the external fuel pumps that are an option when you buy the kit. Do yourself a favor and spend the money on an in-tank fuel pump setup and you will be saving money in the long run.