The number of leaves isn't the whole picture; the spring rate is also determined by the leaf thickness and some details about the shape of the springs. Not to mention that Mopars use highly asymmetrical springs with different rates left to right, and different ratios of stiffness front to rear on each side. A good spring builder will know how to pick out the right ballance, a bad one might just give you two identical springs with a rate that's more or less the same as stock. There's a lot more to this question than just spring rates - the real question is whether he knows his Mopars. Some guys copy the factory spring designs, some do actual testing on cars to see if they've made an improvement, and some just guess or try to copy the spring rate number and ignore the rest.
If he is duplicating the factory 4 leaf setup, you'll have a more softly sprung ride, not a good point for a lowered car. If he's designed this from scratch... do you trust his chassis engineering skills?
