I've mentioned this before, but it sometimes is overlooked. Please keep in mind your truck was designed to use the same diameter tires (and wheels naturally) front and back. If your front tires are a smaller diameter than your back tires, your caster measurement will be reduced. If you've had your front axle bent to lower it or you're doing anything to lower your front axle relative to your back axle, everything changes. There are no real rules on caster...you have to experiment a bit to see what works best, but 4 degrees could be a bit on the high side in my opinion. What Jerry said about having to lift the truck with more caster is right on target and important.
Caster (simply put) places the load point ahead of the contact point of the tire at the road. This causes the wheels to track straight and to return after turning. Correct tire pressure is important. Not using tires which are greatly too wide is also important. A tire greatly wider than the suspension was designed for changes the scrub radius and this isn't adjustable.
Finally too much caster can cause problems aside from making the truck hard to steer. It can increase shimmy and make bumps and road shocks more dramatic. It will cause kingpin parts to wear out more rapidly. It isn't usually a problem with straight axles, but if you have unequal caster, it will cause your truck to pull toward the side with the lower caster. Negative caster will cause: wandering, tendency to bump steer, instability and can give you that sort of "Jeep Wobble" that is nearly impossible to control until you bring the truck to a stop.
Jon
