The share of the system memory dedicated to the GPU can be changed within the BIOS.
On his particular motherboard this option would be located at:
BIOS => Peripherals => GFX Configuration => UMA Frame Buffer Size
He can force several options from 256MiB to 2GiB, or leave it at auto.
With only a total of 4GiB available, I would not recommend setting it to more than 1GiB.