grzesiu.php

<?
    $output .= '<ul class="wiadomosc">';
 	  
    $n = 0;
    foreach($artykuly as $a){
        
        $thumb = get_the_post_thumbnail( $a->ID, 'full', $attr = '' );
        $tytul = $a->post_title;
        $tresc = $a->post_content;
        $output .= "<li".(($n++ > 3 )?' class="twoja_wybrana_klasa"':"").">".$tytul."</li>\n"; 

    }
    
    $output .= '</ul>';
?>