I'm surprised that you don't see the changes between RR3 and 4 as drastic. It's a completely different API, and unsolved a problem which had (admittedly imperfect) solutions -- kicking off data fetching prior to navigation.

I try to resist hyperbole, but if it wasn't for the brand value of owning the name "React Router", it should (or would) have been released under a different package name.