I checked applicake/doorkeeper issues on GitHub. This gem does not support Rails 4 yet, and 0.6.x will not support.
https://github.com/applicake/doorkeeper/pull/200
We won't plan to support versions less than 1.0 for rails 4.
But someone already forked for Rails 4 like this https://github.com/applicake/doorkeeper/pull/213 .
You can use these forks as below.
# Gemfile
gem 'doorkeeper', github: 'kaznum/doorkeeper', branch: 'support_rails4'
Hope to help you.