SELECT touren.lang, touren.id AS tour_id, touren.region_id, touren.tourenthema_id_1, touren.tourenthema_id_2, touren.tourenthema_id_3, touren.tourenthema_id_4, touren.id, touren.titel, touren.kurzbeschreibung, touren.langbeschreibung, 
region.id, region_name, 
tourenthema.id, tourenthema.tourenthema_name AS t_name 
FROM 
touren 
LEFT JOIN region ON touren.region_id=region.id 
LEFT JOIN tourenthema ON touren.tourenthema_id_1=tourenthema.id AND touren.tourenthema_id_2=tourenthema.id AND touren.tourenthema_id_3=tourenthema.id AND touren.tourenthema_id_4=tourenthema.id 
WHERE touren.id='$id'

SELECT
    t.lang,
    t.id AS tour_id,
    t.region_id,
    t.tourenthema_id_1,
    t.tourenthema_id_2,
    t.tourenthema_id_3,
    t.tourenthema_id_4,
    t.id,
    t.titel,
    t.kurzbeschreibung,
    t.langbeschreibung,
    r.id,
    r.name,
    tt.id,
    tt1.tourenthema_name AS t_name1
    tt2.tourenthema_name AS t_name2
    tt3.tourenthema_name AS t_name3
    tt4.tourenthema_name AS t_name4
FROM
    touren t
LEFT JOIN region r ON t.region_id = r.id
LEFT JOIN tourenthema tt1 ON t.tourenthema_id_1 = tt.id
LEFT JOIN tourenthema tt2 ON t.tourenthema_id_2 = tt.id
LEFT JOIN tourenthema tt3 ON t.tourenthema_id_3 = tt.id
LEFT JOIN tourenthema tt4 ON t.tourenthema_id_4 = tt.id
WHERE touren.id = $id

if($erg->tourenthema_id_2 != "") echo "Tourenthema ( 2 ): ",$erg->t_name2,"
";
if($erg->tourenthema_id_3 != "") echo "Tourenthema ( 3 ): ",$erg->t_name3,"
";
if($erg->tourenthema_id_4 != "") echo "Tourenthema ( 4 ): ",$erg->t_name4,"
";

object(stdClass)(16)
{
	["lang"]=> string(2) "DE"
	["tour_id"]=> string(2) "37"
	["region_id"]=> string(1) "1"
	["tourenthema_id_1"]=> string(1) "2"
	["tourenthema_id_2"]=> string(1) "3"
	["tourenthema_id_3"]=> string(1) "4"
	["tourenthema_id_4"]=> string(1) "2"
	["id"]=> string(1) "2"
	["titel"]=> string(4) "test"
	["kurzbeschreibung"]=> string(4) "test"
	["langbeschreibung"]=> string(4) "test"
	["region_name"]=> string(62) "Naturarena"
	["t_name1"]=> string(18) "Familienwanderwege"
	["t_name2"]=> NULL
	["t_name3"]=> NULL
	["t_name4"]=> string(18) "Familienwanderwege"
}

id | tourenthema_name | tourenthema_beschreibung | lang

2 | Familienwanderwege | Bequeme, breite Wege... | DE
3 | sdfh | we45u w4s57 zhdsgh | DE
4 | ttttttttttttttttttttttttttt | ttttttttttttttttttt | DE

SELECT
    t.lang,
    t.id AS tour_id,
    t.region_id,
    t.tourenthema_id_1,
    t.tourenthema_id_2,
    t.tourenthema_id_3,
    t.tourenthema_id_4,
    t.id,
    t.titel,
    t.kurzbeschreibung,
    t.langbeschreibung,
    r.name,
    tt1.tourenthema_name AS t_name1,
    tt2.tourenthema_name AS t_name2,
    tt3.tourenthema_name AS t_name3,
    tt4.tourenthema_name AS t_name4,
FROM
    touren t
LEFT JOIN region r ON t.region_id = r.id
LEFT JOIN tourenthema tt1 ON t.tourenthema_id_1 = tt1.id
LEFT JOIN tourenthema tt2 ON t.tourenthema_id_2 = tt2.id
LEFT JOIN tourenthema tt3 ON t.tourenthema_id_3 = tt3.id
LEFT JOIN tourenthema tt4 ON t.tourenthema_id_4 = tt4.id
WHERE touren.id = $id

SELECT 
    		t.lang, 
		    t.id AS tour_id, 
		    t.region_id, 
		    t.tourenthema_id_1, 
		    t.tourenthema_id_2, 
		    t.tourenthema_id_3, 
		    t.tourenthema_id_4, 
		    t.id, 
		    t.titel, 
		    t.kurzbeschreibung, 
		    t.langbeschreibung, 
		    r.id, 
		    r.region_name, 
		    tt.id, 
		    tt1.tourenthema_name AS t_name1, 
		    tt2.tourenthema_name AS t_name2, 
		    tt3.tourenthema_name AS t_name3, 
		    tt4.tourenthema_name AS t_name4 
		FROM 
		    touren t , tourenthema tt
		LEFT JOIN region r ON t.region_id = r.id 
		LEFT JOIN tourenthema tt1 ON t.tourenthema_id_1 = tt.id 
		LEFT JOIN tourenthema tt2 ON t.tourenthema_id_2 = tt.id 
		LEFT JOIN tourenthema tt3 ON t.tourenthema_id_3 = tt.id 
		LEFT JOIN tourenthema tt4 ON t.tourenthema_id_4 = tt.id 
		WHERE t.id = $id