Here, I will show just a few features to be included in Rails 4.
Rails 4.0 depends on Ruby 1.9.3 (and later). Some parts of Rails 4 code use ruby 1.9.3 specific syntax, so, old versions of ruby will not be supported, if you are using one of them, please, update.
Polymorphic from generators
We will be able to generate resources specifying a polymorphic association directly from generators, for example