Correct the version number to 4 in guides[ci skip]

Reference To:  [The Rails Initialization Process](https://github.com/rails/rails/blob/17c29a0df0da5414570b025b642e90968e96cddc/guides/source/initialization.md#the-rails-initialization-process)

> This guide explains the internals of the initialization process in Rails as of Rails 4. It is an extremely in-depth guide and recommended for advanced Rails developers.