From the description of the problem (i.e. took one stick of 4GB), I believe he/she is using two sticks of 4 GB modules.
Is the voltage requirement for both memory modules the same? Sometimes it is very difficult to ascertain the voltage information for RAM. I had a similar problem just yesterday, and it turned out to be a voltage discrepancy.
I solved the problem by buying another identical memory module. I know it costs more this way, but 4 GB is fairly inexpensive nowadays.