How to change default port of a Rails 4 app?

Append this to config/boot.rb:

require 'rails/commands/server'

module DefaultOptions
  def default_options
    super.merge!(Port: 3001)
  end
end

Rails::Server.send(:prepend, DefaultOptions)

Note: ruby >= 2.0 required.

Quick solution: Append to Rakefile

task :server do
  `bundle exec rails s -p 8080`
end

Then run rake server