With only two sticks installed, try going into the bios and upping the DRAM voltage by .05v. Save settings and shut down. Install the other two sticks. Try to POST. If no love, then shut back down, pull the two sticks out, and try upping again by .05v. Save settings, shut down, Install the two sticks again and try to POST.
If you get no love by the time you've added .2v total, probably isn't going to happen without an OC on the CPU. Overclocking the CPU may not even be possible depending on the CPU and the motherboard. OEM motherboards rarely, if ever, allow overclocking as the OEMs don't want users messing with things and creating situations where they have to deal with unnecessary warranties.
Might not happen anyhow if all the sticks were not purchased in a matched/tested set. There are NO guarantees of compatibility between modules EVEN if they have the exact identical part number when purchased separately. Too many variables. Different batches from different production runs can have memory chips that don't play nice together from one batch to the next or may have slightly different sub settings. Best bet is always to run two modules that add up to the amount of RAM you desire and are purchased together in a matched set.
Further, sometimes four modules, which adds increased voltage strain to the memory controller, simply won't all run at a specific speed and may need to be downclocked in order to do so. Motherboard and CPU manufacturers never guarantee that a particular speed can be run when four modules are used. USUALLY it can be made to work, but not without a fair amount of tweaking settings in the bios and SOMETIMES not without an OC on the CPU depending on the chipset, CPU and memory configuration.
Other times, it just won't work, period, without all modules being tested and confirmed to run together which is how you get them in a matched set. Occasionally, even then a set of modules will refuse to run together especially if it's a quad set being run in a dual channel motherboard.