SOQL
SELECT Name, AirRegulator__c, AmountReview__c, BatteryCapacity__c,
ChargingTime__c, Length__c, NumberUnits__c, Rating__c, TankSize__c,
(SELECT UnitPrice FROM PricebookEntries)
FROM Product2
WHERE Id IN (SELECT Product2Id FROM PricebookEntries WHERE UnitPrice = 1)
LIMIT 4
Error
sObject type 'PricebookEntries' is not supported. If you are attempting to use a custom object, be sure to append the '__c' after the entity name. Please reference your WSDL or the describe call for the appropriate names.
PricebookEntries is not custom object