I agree on weight and yes, a 4000# boat/trailer should be fine without a WD hitch.
Although the tow rating is 9500#, you must subtract all the gear, people, etc. in the truck and in the trailer, plus the wet weight of the trailer. See this thread for a GCVWR calculation I did for my future setup.
Towing, continued...
My father-in-law tows a small fifth wheel (you can not stand up in the front overhang) with a Ford F150 (3 years old, when he upgrades, I'm going to try to talk him into the Titan!!) SuperCab with a 6 1/2' bed. I think the 5 1/2' bed would not be long enough, even for his 5th. Believe it or not, he tows with the Ford 4.6L engine! That explains why I could accelerate up hills with my Honda Odyssey towing a 2200# pop-up (and a wife, 3 kids, dog, roof-top box, .....) and he could barely maintain speed towing his 5000# 5th.