rescue_from ActionController::RoutingError in Rails 4

In application_controller.rb add the following:

  # You want to get exceptions in development, but not in production.
  unless Rails.application.config.consider_all_requests_local
    rescue_from ActionController::RoutingError, with: -> { render_404  }
  end

  def render_404
    respond_to do |format|
      format.html { render template: 'errors/not_found', status: 404 }
      format.all { render nothing: true, status: 404 }
    end
  end

I usually also rescue following exceptions, but that’s up to you:

rescue_from ActionController::UnknownController, with: -> { render_404  }
rescue_from ActiveRecord::RecordNotFound,        with: -> { render_404  }

Create the errors controller:

class ErrorsController < ApplicationController
  def error_404
    render 'errors/not_found'
  end
end

Then in routes.rb

  unless Rails.application.config.consider_all_requests_local
    # having created corresponding controller and action
    get '*path', to: 'errors#error_404', via: :all
  end

And the last thing is to create not_found.html.haml (or whatever template engine you use) under /views/errors/:

  %span 404
  %br
  Page Not Found