I would either state the entire time that you've used the iPhone 4 (total equaling this unit plus the other one(s) that had to be taken to the genius bar) and just say it's in like new condition. Or just say that it was recently replaced by Apple and is in brand new condition. Neither is dishonest.
As a buyer I worry most about water damage, physical condition, and warranty as long as the phone is functional. So for warranty purposes, it is simply the total time you've had the iPhone 4 since the first one you bought even if replaced by Apple. Physical condition you would be saying is "like new" since it's not new and I assume you will keep it in a case till you sell it. And you'll show that there is no water damage and that it's working.