So you already lowered your truck 4 inches in the back with hangers and shackles and now you want to go a little bit lower. Well I hate to tell you this but it's not going to be that easy. The next step in lowering the back is a flip kit which will put the axle on top of the leaf pack. And if you do that without putting the factory hangers and shackles back in you're going to end up with about 10 inches of drop in the back which will make it undriveable.
But, you might be able to get a little bit more without a lot of work, if your truck has a factory overload leaf you can remove it and your truck will come down by the thickness of that leaf, usually about half an inch. You can also check the shackles in case they have an extra hole in them that you can drop them down a bit farther but you've probably already looked at that.
So then you really do need to put it back to factory height and then do a flip kit.