I think the 4gb should be standard of course on the 960 non ti but for those on a strict budget,a 2gb 960 really wouldn't hurt either as a option to save money.Something like a 768mb 460.
I just see some games tank so hard on on cards like the 770 on current new games,4gb wouldn't save a 960 non ti in single card usage and that is assuming the 960 non ti could be as fast as a 770.
Find me a recent game seriously that can run 50+fps minimums on a single 680/770 at 1080p,hit pass 2gb vram on a 4gb model and a reviewer recommending it as a excellent go to card.Not a junk pile console port either but a quality port.I say good luck on that search.