application_controller.rb

class ApplicationController < ActionController::Base
  # see also: http://stackoverflow.com/questions/2385799/how-to-redirect-to-a-404-in-rails
  def not_found!
    raise ActionController::RoutingError.new('Not Found')
  end
end

あーそうか。真面目に render :status とかやるから面倒なのか。