My buddy recently installed the Pro-Flo 4 on his 440. The main reported advantage is the nicely updated programing. They really upgraded how well the system self learns and adapts to ongoing operations. It's reportedly levels above their Pro-Flo 3 set-up.

As mentioned in the first response though, if your system is really well dialed in, there will be little noticeable difference. The main difference between the two should be the amount of tuning you had to do manually to get your current system dialed in, especially hot start and the more complicated tuning issues. Where as the Pro-Flo 4 pretty much does all that tuning using its self learning and more refined initial set-up tables and strategies.