SELECT 
  a.modifier, 
  a.modifier_type, 
  a.variant_id 
FROM 
  cscart_product_option_variants a 
WHERE 
  1 
  AND a.variant_id IN (4123, 4143, 4145, 4147)

Query time 0.00161

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "a",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 4,
          "filtered": 100,
          "index_condition": "a.variant_id in (4123,4143,4145,4147)"
        }
      }
    ]
  }
}