小弟目前僅用 3G，所以無法回答是否有相同狀況發生。

不過倒是有網友與您一樣：

4Gb memory issue with Bus Desktops and WinXP (HP 討論區)
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=894830&hpweb_printable=true

裡面有位 Scott Wiginton 提到「...However, this will not really solve your problem. The upper memory area is reserved for things like memory mapped IO space, PCI configuration space, PCI-Express extended configuration space, ACPI tables, various buffers, video frame buffers (if using embedded video), etc. Each of these consume chunks of memory. When you have 3GB or less of physical memory, you do not notice this because the top of addressable (or virtual) memory is consumed. The system pages memory in and out as needed. With 4GB of physical memory, you see that is used and this space is unavailable for Windows to use...」

4 GB memory not recognised in XP Pro? (MCSE Software Forum)
http://www.mcse.ms/message1457725.html