We were there just a few months ago for 10 days. I paid for the lagoon tower room using their hawaii 50th celebration 5th nite for 50 dollars plus get $50 resort credit (so it's really like 5th nite free). (Since it was 10 days the wanted to only give me the resort credit once, so I made them split it into two reservations and the front desk person had to do that to give it to me twice).
Anyway, I am a single parent with 4 kids and the reservation desk gave the runaround at first then said ok per a supervisor. With two adults they might have had more issue. I think I'd just fly under the radar screen and not mention reserving for the 4th kid. The catch is that the rooms are not queen but full size beds so 3 to a bed is very tight. They make sure the couch is too small to sleep on.....