And getting more would be really pushing it on a Deneb, and could possibly create even more problems.
Running with four DIMM's should be no problem, whether is's Heka, Rana, Deneb.. the IMC's are fine.
Some people find they may have a weak IMC, and thus higher clocks are not as attainable as others.
With the Deneb, the IMC does not like more than 2 sticks of ram. That is why he said it would be taxing on the Deneb proc.
I currently have 4 X 2gb in my system and I had to downclock the speeds of the ram to get over seize ups and some other unflattering effects from all those DIMMS.
More ram is always good to have, for sure, just make sure with Deneb, you say increase ram amount while maintaining only 2 sticks to avoid any plug and play issues.
It is likely you haven't setup your RAM correctly, timings, voltage, combination of the three, etc; or are trying to push your RAM or other clocks too far. Beyond that the last issue would be incompatibility with your motherboard.
There is nothing wrong with running four DIMM's. This is a common misconception which was grabbed on by people who weren't overclocking their systems be cause people who were clocking their's higher (including CPU/NB, RAM, and CPU all simultaneously) were found reporting that some overclocks were unattainable with four DIMM slots populated due to over-stressing the memory controller.
Even mild clocks should never be a problem. I have successfully had 4x2GB DIMM's of DDR3 1600 @ 1600 @ 7-6-6-21-27 running on a C2 720BE for over a year before I sold a kit to purchase other components.