Nvidia are planning to discontinue the 2GB versions of GTX 960 GPU and only bring 4GB versions in the future.
This decision is mostly due to two factors, first Nvidia want to compete with AMD’s R9 380 GPU, which also comes in both 2GB and 4GB configurations with a larger 256-bit memory bus. The second thing is new games high texture size, which is not good for a 2GB card in maximum settings at 1080p for some games.
As this change from 2GB to 4GB of VRAM will not make much or any performance differences in most games, but some games with high-resolution texture will get more VRAM to eat at the end give more FPS. Also this make the GTX 960 more capable of handling future games.