I’ve just had the same problem:
http://digitalconsumption.com/forum/Gigabyte-GA965PDS4-with-3GB-or-4GB-RAM-Memory-Hole
Upgraded to 4gb and only seeing the benefit of half of my upgrade.
The galling thing is that I bought the motherboard because it specifically stated that it supported 8GB of RAM. Gigabyte support just told me that no memory remap function is available.