<?php
        $thisCates = get_the_category();
        $thisCat = $thisCates[0]->cat_ID;
        $args = "orderby=rand&showposts=4&cat=$thisCat";
        query_posts($args);    
        if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
        Шаблон поста
        <?php endwhile; else:
        endif; wp_reset_query(); ?>