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As mentioned below, there is that official upgrade guide. It is not comprehensive, as mentioned in the post. Should get you going, though. We'll be continuing to work on it now that there's an RC out.

I personally have reviewed (but am not getting a cut of) http://www.upgradingtorails4.com/ and it's pretty good.

There is one here:

http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.ht...

I highly recommend watching these free videos from Code School: http://rails4.codeschool.com/videos.