Compare this image from my system to that from the first post in this thread. As you can see, they are both 64-bit systems with 4GB of RAM. Mine, however, is showing the full amount as being usable (it only says xGB usable if less than the installed amount).
Since you have already checked the settings I mentioned in my earlier post about Maximum memory:
(you might like to double check),I can only think that it might be a BIOS setting.