Look at the label on the PSU for the Brand Name & Model Number.
Brand & Specs of the RAM?
Mixing RAM brands/specs commonly results in problems and filling all the RAM slots can cause problems as well.
Mismatched RAM can work periodically and then be troublesome.
When RAM is mixed, the faster RAM is reduced to the speed of the slower RAM.
If the 2x2GB runs with no problems I would stay with it. 4GB is plenty unless you're doing serious graphics/CAD work.
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