How do you issue a 404 response from a rails controller action?

This seems good...

# Rails 2 and below
render :file => "#{RAILS_ROOT}/public/404.html",  :status => 404

# Rails 3 and up
render :file => "#{Rails.root}/public/404.html",  :status => 404

You can also

raise ActiveRecord::RecordNotFound

exception.

In the ApplicationController define a method like:

def render_404
  render status: 404, layout: false file: Rails.root.join("public/404.html")
end