SELECT 
  cscart_product_prices.product_id, 
  MIN(
    IF(
      cscart_product_prices.percentage_discount = 0, 
      cscart_product_prices.price, 
      cscart_product_prices.price - (
        cscart_product_prices.price * cscart_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  cscart_product_prices 
WHERE 
  cscart_product_prices.product_id IN (
    4718, 4793, 4774, 4776, 4775, 4078, 4779, 
    4777, 4792, 4787, 4858, 4786, 4790, 
    4778, 4856, 4860, 4796, 4797, 4795, 
    4794, 4857, 5006, 4878, 4112
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "range",
      "possible_keys": ["usergroup", "product_id", "lower_limit", "usergroup_id"],
      "key": "product_id",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 49,
      "filtered": 97.95918274,
      "index_condition": "cscart_product_prices.product_id in (4718,4793,4774,4776,4775,4078,4779,4777,4792,4787,4858,4786,4790,4778,4856,4860,4796,4797,4795,4794,4857,5006,4878,4112)",
      "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}