Tim is correct, the 4100 uses a much wider and shorter spring, 46" x 2 1/2" spring, where the lighter 3100 trucks use, 54" x 1 3/4" springs. What truck model do you have?
New will not be softer, you are going to have to down size or add weight to the truck. Poly-liner or teflon rub pads helps quite a bit and removing some leafs, but as you remove leafs, you remove load capacity.
Joe
