Here are some pros and cons based on my experience:
1. A four wheel drive equipped vehicle weighs more than a 2WD. The extra elements such as a transfer case and added drive shaft add weight. Let's say you purchased a 2WD and a 4WD that were exactly the same truck. Suppose the Gross Vehicle Weight Rating is 9,000 pounds. If the 2WD truck weighed 7,000 Pounds, you would be able to carry 2,000 pounds on that truck. That's called payload. It consists of all weight you put in the truck including driver and passengers, cargo, and trailer pin or tongue weight. If you took the same truck in 4WD, it might weigh 7,500 pounds, but would still have the same 9,000 pound GVWR. So on the 4WD truck, you'd only be able to carry 1,500 pounds (9,000-7,500).
2. I've been in situations where I was trying to back my FW up a driveway to a campsite that was fairly steep and covered in gravel. My truck tires started spinning. I put it in 4WD, and backed in with no problem. If I'd only had 2WD, I would have needed an alternate plan. I've also been bogged down in mud, and used the 4WD to get out.
3. 4WD vehicles are a little more expensive to maintain (but not much). Also, they usually cost more when you make your initial purchase. However, most 4WD trucks will have a higher resale value when you eventually sell it.
4. Normally, 4WD will not give you added towing capacity.
Jim