When is a WDH neccessary

A WD is needed when the tongue weight exceeds the manufactures hitch rating for weight carrying ability, when the rear of the tow vehicle sags so much that headlights will not aim properly, when the steering feels light or out of control, when you are exceeding the rear axle or tire loading ratings of the tow vehicle, or when the the trailer feels like it is taking control of the tow vehicle every time you hitch it up, or a combination of all or most of these.

My feelings are that you will probably not need one for the trailer you are considering BUT keep in mind that tongue weights are usually much heavier than described by the trailer brochure or specs by the time you are ready to go camping. Also don't know what you mean by a "considerable smaller truck".
A truck like a Ford Ranger may need one while a F150 probably would not. Many people are quite surprised when they eventually get around to actually weighing the tongue of the trailer.
Barney

Not exactly the route I went but the down sizing was. I had a Motor Home that I was using as my home base. I had bought a 17 foot Casita single axle trailer to travel with. I'm single so room was not a problem. Used the money from the sale of the MH and bought a used 40 foot 5th wheel trailer and had it hauled to my lot, probably never to be moved again by me.

So that left me with a 17 foot Casita/ Tacoma pickup. As far as WDH is concerned, that is a question only you can answer. What I can tell you is the rig rides smoother with than without. I use an Andersen WDH which also has it's own built-in sway control.

The only reason I use one is to keep the headlights aimed down.
No need to blind oncoming drivers.

A WD hitch is recommended for any trailer >5000 pounds. It may be indicated for a lesser trailer depending on your tow vehicle. I use them primarily to prevent overloading the hitch itself; they are not a substitute for a tow vehicle with proper payload and tires to handle the capacity.

I towed my current TT, 3877# GTWR, 23' long tip to stern, a so called 16' actually 18' rounded front to back, with my 2008 Explorer. A WDH was needed from the squat and steering control.
Not needed with my new F150 but makes for much better towing, no sway, and levels out the truck.
I think, unless you can get a great deal on the WDH with install from your TT dealer, you should try the new TT on a standard hitch first (assuming within hitch weight specs).
I lost one of my spring bars on a trip from improperly attaching it in a hurry. Found out when setting up camp. Towed home with no spring bars, 70 miles one way mostly on interstates. No probs but sidewinds could be felt at times so I kept the speed at 65mph.

what truck? I would suspect it will be advantages regardless.

Most F150 size and smaller trucks have a receiver rated for 500lbs of tongue weight, or 1000lbs of tongue weight using a WDH.

You need to look at the specs of the truck you are considering.

A 4000lb trailer, is that the "dry" weight or the GVWR?

If the GVWR is 4000lbs, you could conceivably have a 600lb tongue weight (15%), which would necessitate the use of a WDH. Trailers in this range are generally single-axle units with high tongue weights to allow the use of a single 3500lb axle.

If the dry weight is 4000lbs, your trailer will probably end up in the 5000-6000lb range loaded and ready to camp, and will most certainly have a tongue weight exceeding 500lbs, again necessitating a WDH.

In both cases, this is assuming a typical F150 type truck or smaller with a typical receiver with a weight carrying limit of 500lbs tongue weight.

Simple test with YOUR TV and YOUR FULLY LOADED TT to determine the need for a WDH. Hookup your fully loaded TT to your TV and measure the front wheel well height. Raise the TT tongue off the hitch without moving the TV and remeasure the wheel well height.

If the difference between the two measurements is less than 1/4 inch the you could get by without a WDH. If the difference is between 1/4 to 1/2 inch then you should get a WDH. If it's more the 1/2 inch then you absolutely need a WDH.

The best test is to weigh your TV steering axle with the loaded TT attached and a second weighing with the TT NOT attached. If attaching the TT REMOVES weight from the TV steering then you need a WDH.

Rover_Bill wrote:
Snip... If attaching the TT REMOVES weight from the TV steering then you need a WDH.

Not quite. It will ALWAYS remove weight from the front axle. How much is determined by several factors but it will always happen. It may not always have the same effect though.

Given all factors being the same, then putting a weight behind the rear axle of the tow vehicle will always remove weight from the front axle. It doesn't matter how much the front of the truck weighs - the weight will still come off. The owner then needs to decide whether the truck is still within specs or whether WD is needed.
Barney

If your looking for a 4000# trailer then it's a safer bet to find a trailer with a GVWR of 4000# as every trailer I've owned "3" they ended up much heavier than the proposed dry weight and very close to GVWR.

To many advantages to a WDH with antisway to not use one, safety being #1.