I registered just for this problem.
Fallout 4 is consistently using only 2gb to 2.4gb of vram on my R9 370 4gb.
The PC is a completely stock Alienware X51 R2 (with i7 4790 running at 3.6 and 16gb of ram).
I'd really like to get more out of the gpu since I chose it over the Nvidia GTX 960 2gb for the extra vram and a foolish sense of customer loyalty (old pc used a 6770 gpu from amd).
I use GPU-Z to monitor the card.
Thanks for the help, advice, or even complaints about what I have or have not posted.