Ok, I was previously in the “this 4gb patch is rubbish” camp. I never seen a game executable use more than 2gb of ram and something would have to be seriously wrong for it to do so. Even this HD Textures stuff didn’t match as they should be sitting in GPU ram not system ram.
Textures don't move straight from the HDD to the GPU, they have to go into the RAM for loading, decompression and such. Now, imagine if you're moving between cells and the game has to load a lot of new large textures, whilst still keeping the old data about the last couple of cells around, and it's loading and decompressing these textures in multiple threads so has multiple textures open at the same time. RAM usage can go up quite a bit, and it only has to go over the limit once for a few instructions between that one "allocate" and "free" call for everything to come crashing down...