You should be careful with all the paths!
I cannot see "autoload.php" in your file system. In my case I put the content of PHP SDK in the directory "fb", and use the paths (added DIR . into the "define" line in comparison with the example of https://developers.facebook.com/docs/php/gettingstarted/4.0.0)
define('FACEBOOK_SDK_V4_SRC_DIR', __DIR__ . '/fb/src/Facebook/');
require __DIR__ . '/fb/autoload.php';
echo FACEBOOK_SDK_V4_SRC_DIR; //to check if the paths are correct
use Facebook\FacebookSession;
use Facebook\FacebookRequest;
use Facebook\GraphUser;
use Facebook\FacebookRequestException;
<...>
It helped to avoid that kind of the error.