Mobile Intel 945PM Express Chipset

Although the PC has a 64-bit core 2 duo CPU these PCs have a 32-bit northbridge (CPU <-> memory connector chipset) so can only address a maximum of 232 (4GB). Of that 4GB total the upper 1GB is assigned for PCI devices, graphics card memory, and other memory-mapped I/O devices.

Therefore the total useable RAM is around 3GB even if 4GB is installed.

According to the  Mobile Intel 945 Express Chipset Family Datasheet, section 9.2 Main Memory Address Range. figure 13 PCI Memory Address Range:

Intel 945PM PCI Memory Address Range