|Memory that is used by an integrated video system is not available to Windows and is not reported as such.|
A 64 bit OS cannot automatically use all 4GB, even if it is not in use by the video system. You have the same problem as in a 32 bit OS, that is a portion of the address space is shared with memory mapped hardware which must have priority. In a 32 bit client OS there is no solution, but there is with a 64 bit OS.
To use all 4GB of RAM the conflicting memory must be mapped to a region above the 4GB mark. There is usually an option in the BIOS to do this. Since every system is different I don't know what it is called in yours.