Oh, the hidden slot has a 4Gb module? I was thinking it had a 2Gb module.
I would take out all but that one. Power up and check the BIOS to make sure it registers properly (as 4Gb).
Next, add a 4G module to slot 2....wherever that is. Again, check to see if the BIOS recognizes it and shows 8Gb total.
Add another 4Gb module, this time in slot 3. Check the BIOS to see if it registers the expected 12Gb.
Personally, I wouldn't add the 2Gb module, but if all is working well up to this point, you could try putting it in slot 4.
A concern here is potential memory mismatches. The original two modules are different...2Gb and 4Gb...hopefully they are at aleast the same manufacturer and timings. Then your added GSkill RAM, which is perfectly good memory, but it's probably different from the OEM RAM manufacturer. Whenever possible, it's best to have matched modules....different RAM can sometimes cause problems.