I'm basing this on what I've read, and my experience with Rails 3, so take it with a grain of salt. The type of application you couldn't do in the older one? Ea

I'm basing this on what I've read, and my experience with Rails 3, so take it with a grain of salt. The type of application you couldn't do in the older one? Easy single-page apps. Of course you COULD do it, writing your own JavaScript or CoffeeScript. But Turbolinks and the Russian Doll caching give Rails 4 a way to get much of the speed of those custom-written apps and yet stay in Ruby/Rails. I don't know of any type of application that Rails 4 CAN'T do, but it's still Rails and Ruby, so anything computationally intense would probably be better served (ha!) by a Java (or Go, or Scala, or...) framework. The further you get from a CRUD app, the less useful the Rails framework becomes.