Except he is running 64 bit. You should think before you answer questions... Does your motherboard have a that as the ram limit perhaps? It looks like the motherboard has a 4gb memory limit. Perhaps the memory controller has a 32 bit limit which means there's nothing else you can do.
from asus site : 4 x DIMM, max. 4GB, DDR2 667/533/400, non-ECC, un-buffered memory Dual Channel Architecture * Due to general PC architecture, a small amount of memory is reserved for system usage and thus the actual memory size is less than the stated amount.
Your mainboard has a intel 945 class chipset IIRC. MC of this class does not support 64bit, IOW you will still have the issue you're currently having even if you're using 64bit OS.
No, on 64bit Windows all 4GB must be seen without any modding. Only 32bit OS-s need some mods for to be seen all of them.
It's not true. I myself am running Vista x64 right now on my laptop. All 4GB are visible and useable. My guess out of the dark is that his motherboard / bios dedicates the missing ram to, maybe integrated graphics, maybe something else. Probably this makes no sense when i wake up tomorrow