Well I got the information from Microsoft's web page! They actually gave you the limit for a 32bit OS.
HP might have a limit on their motherboard. I believe that the Microsoft web page is correct and there are multiple users out there that are using more than 4GB of ram.
Can you post the model number for your computer?
It is an H/P Compaq Presario CQ5500F w/ Windows Home Premium 64 bit.