Tengo 4 tablas en MySQL para unirme. Ejempl

$select = $db->query("SELECT * 
                        FROM ads 
                       WHERE ad_pic='1' 
                         AND ad_status = '1' 
                    ORDER BY ad_id DESC LIMIT 0,4");

while ($fetch = $db->fetch($select)) {

    $iquery = $db->query("SELECT * 
                            FROM images 
                           WHERE img_ads_id = '" . intval($fetch['ad_id']) . "' 
                             AND img_status = '1' 
                           LIMIT 1");
    $thumb = $db->fetch($iquery);

    $uselect = $db->query("SELECT * 
                             FROM users 
                            WHERE user_id = '".intval($fetch['ad_userid'])."' 
                              AND user_status = '1' 
                            LIMIT 1");
    $ufetch = $db->fetch($uselect);

    $cselect = $db->query("SELECT * 
                             FROM category 
                            WHERE cat_slug = '".safe_func($fetch['ad_category'])."' 
                            LIMIT 1");
    $cfetch = $db->fetch($cselect);         

}

Quiero saber cómo unirlos en una declaración de selección.

ads table

ad_id   ad_userid   ad_category
-------------------------------
1       2           5

images table

img_id  img_ads_id  
-------------------------------
1       1         

users table

user_id     user_name
-------------------------------
2           John

category table 

cat_id   cat_name
-------------------------------
5        Vehicles