That's true of any system. A "real" 4GB that touches the 4.1GB region will be forced to use CPU main memory over the PCIe bus.

So the 0.5GB region is still faster than PCIe IIRC. Not ideal of course, but still better than a bunch of PCIe transfers.

The 970 is a cut-down 980. Where the 980 has 4GB of memory connected to the core through a 256-bit bus to the 8 32-bit GDDR5 memory controllers the 970 only has a 224-bit (aggregate) bus between the memory controllers and the L2 caches. The last eighth of the memory can only be accessed in a bank-switched fashion that blocks access to the primary 3.5GB bank. The primary bank can be accessed almost as fast as the 980's memory (striping across 7 chips instead of 8), but accesses to the last 0.5GB bank can't be striped across multiple chips. Those two effects together make it impossible to treat the 970 as having a full 4GB of general-purpose RAM.