SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (4004, 40526, 3975, 3990, 3998, 3991)

Query time 0.00183

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 6,
          "filtered": 100,
          "index_condition": "cscart_products.product_id in (4004,40526,3975,3990,3998,3991)"
        }
      }
    ]
  }
}