i think the chipset must support at least 8 GB of address space, if this is Intel's chipset, then starting from Intel 965 chipset upwards.
i didn't check your laptop's chipset , but a quick google, i think here (below) is an example of the similar problem in Intel 945 chipset laptop.
DV9000T & 4G RAM revisited [Archive] - Notebook Forums and Laptop Discussion