The 2nd and 4th slots (from the CPU) are paired. Also the 1st and 3rd slots are paired. To run two sticks of RAM, put them in the second slot from the CPU and the furthest slot from the CPU (2nd and 4th physical slot). Enter BIOS and Load Optimized Defaults. Restart and verify that all 4GB are showing enabled. Make sure the RAM sticks are fully seated in the slots. One of the easiest things to do is not get them in all the way.
Once you get that working, put your next 2 sticks in the first and third (physical) slot. Load Optimized Defaults again and try again. 8 GBs can be tricky to get running. But it should recognize all sticks.