Thanks to Amar Raja, we added a feature to check if the gems defined in your app's Gemfile are compatible or not to Rails 4. You just need to fulfill the following steps.
First, go to http://ready4rails4.net/gemfile/new and copy your Gemfile contents:
Then just click the "Check my Gemfile!" button and voilà! You will see a list of the status of your app's gems:
Probably some of the defined gems are not in our database, so please help us and register their status. Thanks! <3