Custaom query with a problem

I’m using this custom query:

$querystr = "
    SELECT $wpdb->posts.ID
    FROM $wpdb->posts
    WHERE $wpdb->posts.post_status = 'publish'
    ORDER BY RAND()
    LIMIT 4
    ";

but it sometimes take 3 posts, other time 4….why ? I’d take always 4 posts random