Quote Originally Posted by ammofreak View Post

1) last OS was Ubuntu 12.04 (32bit)
Clearly you were running a PAE kernel, which allowed you to see and use the memory above 3.2 GB.

There were many versions of the P4 CPU, and as I recall not all of them were 64-bit capable. Moreover, I also recall that there were motherboard designs that would support a P4, even a 64-bit capable P4, but did not provide a 64-bit address bus to the installed memory modules. Note that on such a motherboard, the 32-bit OS with a PAE kernel, using the 32-bit memory bus, can access 4 GB of memory, but a 64-bit OS cannot. I think that's probably your situation. More semi-relevant info here:

http://ubuntuforums.org/showthread.php?t=869060