The reason you "can" tow a 4500 lb boat without weight distribution bars is that generally you must tow boats without WD bars. Boat trailers use surge brakes that depend on the trailer moving forward against the hitch ball -- WD bars block that. To compensate, boat trailers have very light hitch weights and supposedly their low profile makes them less sensitive to lateral forces (ie yaw/sway). That said, I used to tow a 2 Ton ocean going deep Vee hull 20 footer with an E350 Ford Window van. Highway instability was a real pita, so I had a genius engineering type at Garges Trailers in Inglewood, CA install sliding sleeves on my trailer's A-frame and weld my bar chain lifts to the sleeves. That worked for me, but I have never seen the set up elsewhere.
Absolutely yes to WD bars and anti-sway provision. Go for a Class IV receiver and ball mount. In your set up, with its relatively light TW, I would give consideration to the Equal-i-zer hitch system.
