The Tech Report published a review on the Nvidia admits, explains GeForce GTX 970 memory allocation issue A quote from the article:

We've been tracking an issue with GeForce GTX 970 memory use for a little while now, most notably via a thread in our forums. Some GeForce GTX 970 owners have noticed unusual behavior from these cards compared to the GTX 980. Specifically, the GTX 970 sometimes appears to allocate less than its full 4GB of memory in cases where the GTX 980 does. Also, when asked to go beyond 3.5GB using a directed test, GTX 970 memory bandwidth appears to drop.

 Nvidia admits, explains GeForce GTX 970 memory allocation issue @ The Tech Report