Haven't lowered my truck yet but I'm assuming that a set of shocks would cure the problem and you should probably go ahead and replace the stock bump stop with an old piece of tire or somthing (it is explained in the TECH Section).
I dunno about 4inch... but I have a 2 inch blocks on mine and had the same problem.. All I did was change the shocks to Bilstien and now fine.. I am sure the slammed Taco would say use Toxic shocks from what I am hearing... hey I would like to know too.. I plann dropp it even more... Will air bags be good for performance handling???
Definately remove the stock bump stop. You are probably sitting on it or close to it giving you no suspension travel. Refer to the tech section for an easy and cheap fix. Try this first, then get better shocks if you want to.