They essentially took the basis of FC3 and actually made it fun IMO. There are so many improvements in from the weapons (D2 is a FUCKING BEAST), the gyrocopter, the grappling hook and the skills (Elephant Rider and Vehicle Takedown!).
It also helps that a lot of the side missions are more or less essentially versions of the core thing it does best - letting you tackle something in any way you want. There's still collectibles and shit but there's a lot to enjoy as well.
Regardless though it's a damn fun theme park ride and I'd recommend even haters of FC3 to give it a go, maybe in the next 6 months when you can get it for a price you're willing to take a chance on
I don't know if I'd want another game like FC4 (and i'm worried about Ubisoft beating it into the fucking ground) but I can't deny I've had a blast with it so far.