application_controller.rb

def render_404(exception = nil)
  if exception
    logger.info "Rendering 404 with exception: #{exception.message}"
  end

  respond_to do |format|
    format.html { render :file => "#{Rails.root}/public/404.html", :status => :not_found }
    format.xml  { head :not_found }
    format.any  { head :not_found }
  end
end