bikendan lists the big differences.
But I disagree that trucks always have better payloads than vans.
Heavily decked out trucks have lower payloads. Pay close attention to the yellow door jamb sticker when you look for a new vehicle.
You don't need much of a truck to tow 4000#. If you want something smaller than a full size pickup, check out the Chevy Colorado, you can even get it in a diesel which is great for towing. Or in full size, the RAM Ecodiesel would be an awesome tow vehicle. But again, watch the PAYLOADS. They can be low in diesel trucks especially.
And, hey, even a SUV rated at 5000# tow capacity could pull 4000# when properly equipped. How much of a tow vehicle you need depends a lot upon how much towing you will be doing, and whether the vehicle will also be used for commuting or whatever.