AHA! You da man! It was your second solution...my "max memory" box was indeed checked, and in it was a value of 0.
I simply unchecked it, rebooted, and it displays the full 4gb!
Thanks so much!
If you're *sure* you're running the 64 bit version of the OS, from here I see two possibilities. The first is your motherboard may have a limitation - either by design or by Bios setting. Check that.
For the second: Click the start orb, and type in msconfig. Right click the msconfig app to run it as Administrator. That brings up the configuration utility. There is a setting for Max Memory - Make sure that's unchecked/blank.