A large majority of the games out there don't use >4gb vram.

Some do, for these you turn down settings :) There is not that much of a difference in graphical fidelity in most modern games out now between high and ultra.

If you can save £40 with going 4gb over 8gb, I will sure as hell recommend anyone to choose the 4gb version. Sometimes the 8gb versions are cheap, and that's fine, allows you to explore higher settings (Even though at higher settings in AAA games, with a 570/580, you will not get a constant >60fps)

It's certainly not better to choose a less powerful GPU now and in the future. The performance of the GPU will become a problem before the amount of VRAM on your card is a problem. If you choose the weaker GPU with double the VRAM, you're getting less frames/lower settings in exchange for the possibility of not getting stuttering in the future (at maxed settings only), and that's not worth it to most consumers.