You likely did hit a limit. On my old device, which had a stock density of 160, it was possible to go as low as 120 without an issue, and could easily do 240. I personally haven't attempted to change the DPI on the S4 because the icons are plenty large enough as it is.
My personal recommendation is to not bother using the Touchwiz launcher and install Nova instead, rather than playing with DPI settings. Because as I mentioned previously, everything changes size when you change the density. It's not just the icons that will change size, but the text as well.