.:Consulta por Agrupación:.

[quimfv]

Código MySQL:

SELECT top2.products_id,tosh2.orders_id,SUM(top2.products_quantity)
FROM toc_orders_products top2 LEFT JOIN toc_orders_status_history tosh2
ON top2.orders_id = tosh2.orders_id
WHERE top2.products_id=20
AND (tosh2.orders_status_id = 4 
              OR tosh2.orders_status_id = 5 
                      OR tosh2.orders_status_id = 6 
                              OR tosh2.orders_status_id = 7)
GROUP BY top2.products_id,tosh2.orders_id
 

Mira si así ves mas claro lo que te esta haciendo

SELECT top2.products_id,tosh2.orders_id,SUM(top2.products _quantity)
....
GROUP BY top2.products_id,tosh2.orders_id

Esto suma products_quantity para cada combinacion de products_id,orders_id en tu caso como limitas por top2.products_id=20 te dará la suma para cada orders_id distintos relacionados con top2.products_id=20.... si solo sale un numero serà que products_id=20 solo tinen un orders_id donde ademas orders_status_id es 4,5,6 o 7.

AND (tosh2.orders_status_id = 4
OR tosh2.orders_status_id = 5
OR tosh2.orders_status_id = 6
OR tosh2.orders_status_id = 7)

es igal a AND tosh2.orders_status IN (4,5,6,7) mas comodo no?