sierrax
+1y
A 4 inch block with a mono leaf in the back is just about the most unsafe thing that I have ever heard. Do not do that!
Keep the 3 inch block you have in the back and take your leafs to a spring shop and have them de arched. They may take one of the leafs out when they rearch them. I haven't done it in years but if I can remember right they charge an hours labor to do it. That is the cheapest and safest way for everyone to get a big drop in the back.
Personally I would never run a 4" block, it puts the rear end way to high above the leafs. And remember to re torque your U bolts after a few days.