SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_products_categories AS product_position_source ON cscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 4 
WHERE 
  cscart_products_categories.product_id IN (
    100147, 100146, 100145, 100000, 99981, 
    99977, 99976, 99975, 99974, 99961, 
    99926, 99925, 100304, 100253, 99999, 
    99998, 99997, 99897, 99896, 100294
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00059

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories range PRIMARY,pt pt 3 26 Using where; Using index
1 SIMPLE cscart_categories eq_ref PRIMARY,c_status,p_category_id PRIMARY 3 toolboxdigitalshop_dev.cscart_products_categories.category_id 1 Using where
1 SIMPLE product_position_source eq_ref PRIMARY,pt PRIMARY 6 const,toolboxdigitalshop_dev.cscart_products_categories.product_id 1

Result

product_id category_ids position
99896 4M 0
99897 4M 0
99925 29,4M 0
99926 29,4M 0
99961 4M 0
99974 29,4M 0
99975 29,4M 0
99976 29,4M 0
99977 29,4M 0
99981 4M 0
99997 4M 0
99998 4M 0
99999 4M 0
100000 4M 0
100145 4M 0
100146 4M 0
100147 4M 0
100253 4M 0
100294 4M 0
100304 4M 0