4x4
4x4
You need 16 GB of RAM, but your current setup has two DDR3 2x4 modules. When buying two additional 4 GB sticks, make sure you allocate the first slot to each stick and verify compatibility with your system.
Frequency isn't important, but the IMC will process all sticks at the pace of the slowest one. Aim for matching CAS latency as recommended.
If you don’t locate a matching RAM, you can adjust them manually in the BIOS. Aim for the smallest configuration for each timing requirement... (meaning if one struggles with one latency type while another performs poorly, both will suffer too!) (It’s best to choose one offering the closest timing options.)