application.rb

module MyApp
  class Application < Rails::Application
    require Rails.root + 'lib/custom_public_exceptions'
    config.exceptions_app = CustomPublicExceptions.new Rails.public_path
  end
end

custom_public_exceptions.rb

class CustomPublicExceptions < ActionDispatch::PublicExceptions

  def call(env)
    status      = env["PATH_INFO"][1..-1]

    if "404" == status
      # handle just 404 in our routes
      MyApp::Application.routes.call env
    else
      super env
    end
  end
  
end

errors_controller.rb

class ErrorsController < ApplicationController
  skip_before_filter :authenticate_user! # if using Devise

  def not_found
  end

end

routes.rb

Geknowm::Application.routes.draw do
  match "/404", :to => "errors#not_found"
end