Basically a 4" lift is no different then a 4" drop when it comes to the panhard bar. Techinically in an ideal world the bar should run parallel with the center of the diff and the ground.When you lift a truck 4" the bar runs uphill from the diff mount to the frame bracket,and pulls the diff sideways towards the passenger side,when you drop a truck 4" the bar runs the opposite way,it's downhill from the diff mount to the frame mount,and again pulls the diff towards the passenger side roughly the same amount as the lift did. If a bar works for a 4" lift,it'll work for a 4" drop,if it's a straight bar.A dogleg bar is another story,but can usually be made to work by flipping the bar around in most cases.