The general rule of thumb is with a 4" suspension lift, a dropped pitman arm or raised steering arm is recommended, but not required.
With a 4" lift and no steering correction, you will have uneven steering from left to right. In other words, you can turn sharper one direction more than the other direction.
When going over 4" it is pretty much necessary to use a dropped pitman arm or raised steering arm.
When you get your lift, it should tell you if it is needed and what size is needed.
Another form of correction is a steering block, that could also be used instead of a pitman arm or steering arm.