We tow a boat that in 'normal' trim on the trailer is about 4000lbs. We often end up putting all of the 'stuff' like coolers in the boat and then putting 5-6 adults in the truck, so that would add another 1000lbs to normal load easily.
We'll be pulling the boat again today.
I've probably used the Pathfinder to tow this rig about 7500 miles since '06 w/ no problems at all. I did add airbags after the first season, they really are a good idea.
The only times I've needed 4wd (out of a LOT of launches) have been the time someone had a breakdown on the ramp about 25 feet from the water line and got oil all over the ramp and the time a ramp had washed out and the trailer 'fell off' the end of the ramp and dropped over about a 1 foot ledge. These two times it was WELL worth being able to pop into 4wd. Where I use it more often is in a grass field/hill that I park the boat on at a near by campground, I need 4wd to get up the hill w/ the boat on the back w/o spinning.
I actually get my best fuel economey if I run OD off at about 72mph. That keeps the revs up so the torque convertor doesn't unlock (Kentucky has long/big hills on the interstate). Basically if I run 60-65 I end up having the torque convertor unlock, if I run 72-75 it stays locked and I have basically exactly the same RPM on a hill at both speeds. I can do about 14mpg at that speed, about 11.5 at 65mph.