"Always insert the memory modules into the GREEN slots first, and it is strongly recommended not to insert the memory modules into the PURPLE slots while the GREEN slots are left empty."
1 and 3 are your green slots. 2 and 4 are your purple slots. You use the green slots first, 1 and 3. You are to use the green slots first and latter if you add more ram then use the purple slots, 2 and 4. I have 2 rigs. 1 has 4 sticks of 512 so of course I use all 4. My other PC has 2 sticks of ram, slot 1 and 3. Hope this helps.