You might check your BIOS settings. When the computer boots up you'll have to hold (some key - the delete or some function key, you can google it. it's by motherboard) to take you into the BIOS menu if you aren't familiar. That all 3 sticks are independently good is a good sign. The most likely thing is some setting you have to change so your motherboard knows you're running x64 instead of 32 (which only registers 3 GB). If you had the chance to try 2 sticks of 2 GB each would be nice to see if it's the 4th memory channel in your motherboard that has a problem or if the problem is it's going above 3GB (even 1 stick of 4GB would work). If your system lets you, you could try 1 stick in the first slot and 1 stick in the 4th slot, or a stick in slots 2 and 4(though a lot of boards don't like that, so it may fail just on that.) You might also see if there's a dual channel setting to turn on or off or hyperthreading to turn on/off.
Just thoughts... good luck.
hmmmm.... correction... I just checked your computer specs, and it appears from what I read your computer won't support over 1GB per slot. if that's so, then you can only check by trying a 1GB in several slots but not going over 3GB total... so put a 1 GB stick in say slot 1,2,4 and see if it works fine. (that way you're at 3GB). as well as what leithanne mentioned about mixing up new and old and seeing if they play well.
Edited by LFos42, 15 August 2015 - 10:12 PM.