Due to Intel 945G chipset limitation, it just can support under 4GB momory addressing.
Because PC architectural requirements such as motherboard resources and OS limitations,desktop platforms using 4GB or above memory may be unable to take full advantage of all memory populated on the system. The available memory may report approximately 3.12 GB or less depends on system configurations.
To take advantage of the system memory 4GB or above installed, the computer must meet the following requirements:
• An x64 (64-bit) version of Windows system.
• The chipset must support at least 8GB of address space.
• The BIOS must support the memory remapping feature.
• The CPU must support the X64 instruction set. The AMD64 CPU and the Intel EM64T CPU support this instruction set.
In addition, the 32-bit versions of Windows will reserve a portion of memory for critical system functions such as chipset, BIOS flash, PCI Express, etc. If you have 4GB installed, Windows will indicate less than 4GB is available.
For more information, please refer to Microsoft knowledge base: