Log in/out specific user with ID

I am trying to get user ID from SMF forum cookie and log that user in wp.

This is the plugin that i created:

add_filter('authenticate','custom_login_check', 100);
function custom_login_check($user) {

if ( is_a($user, 'WP_User') ) { return $user; } 

$user = new WP_User(4);
ob_start();
include 'kuki.php';
ob_end_clean();

$user = $member_id;

return $user;

But i am getting error when trying to use wp-login.php.

Call to a member function has_cap() on a non-object in C:\…\wp-login.php on line 598