Some computers even with this solution, still doesn't work the 4 GB's.
And its good to remember you will only see your 4GB with a 64x S.O
I was having a similar problem last year when i decide to upgrade my computer to 4GB but having just 2 GB working.
I have tried almost every solution that you can find on the internet for this problem, and nothing works for me
Then i decide change my motherboard and i was able to use the 4 GBs
Later, i put more 4 GB and now i have 8GB and still have no problems with that
There's some chipsets that doesn't see more that 3,5 GB, and this sux =( This may help too