Into your App\Providers\ErrorServiceProvider at boot method you can register your custom 404 page.

public function boot(Handler $handler, Log $log)
{
	...

	$handler->missing(function($exception) {return \Response::view('errors.missing', array(), 404); });
}

there no longer is App\Providers\ErrorServiceProvider in larave 5, now what!!!!!

To show custom 404 page create class, for example, App\ExceptionHandler

namespace App;

use Symfony\Component\Debug\ExceptionHandler as SymfonyDisplayer;
use Symfony\Component\HttpFoundation\Response;

class ExceptionHandler extends \Illuminate\Foundation\Debug\ExceptionHandler
{
    public function render($request, \Exception $e)
    {
        if($this->config->get('app.debug'))
        {
            return (new SymfonyDisplayer($this->config->get('app.debug')))->createResponse($e);
        }
        else
        {
            return new Response(view('errors/show404'));
        }
    }
}

Then in bootstrap/app.php

$app->singleton(
	'Illuminate\Contracts\Debug\ExceptionHandler',
        // 'App\Foundation\Debug\ExceptionHandler'
	'App\ExceptionHandler'
);

And create view errors/show404.blade.php

Or you could just handle the errors in app\Http\Kernel.php as Laravel already does... Just edit the try/catch in the handle() method.

Read this tutorial http://www.techigniter.in/tutorials/create-custom-error-pages-in-laravel-5/