our outback came with 4 ducts also. It is adequate.
That said, a 13.5K BTU AC unit in the typical 35' trailer is getting pretty marginal when your in direct sun and high temps.
However, being the adventuresome type, I found it is very easy to add more vents to the existing ductwork. I added one in the bathroom into the duct that feeds the bedroom, one more in the bedroom (now 2 in the bedroom) and two more in the living area, one in each main duct. very easy to do and use the aireport (sp?) ducts that can easily be closed off and spread the air horizontally rather than straight down. That plus carefully sealing the ducts with aluminum duct tape to keep any cool air from going into the "attic" was worthwhile. Now it cools down quicker, and it is easy to quickly cool either the living area or BR by closing off vents.