4 GB is more than enough. The only advantages to be gained by 8 GB configurations are found by those who make large numbers of Virtual Machines or use RAM Drive. As these uses are outside of 99% of most peoples computing needs 8 GB is an unnecessary indulgence. In some cases it 8 GB configurations can even slow a machine down and create instability if the motherboard is not of excellent quality. Save some cash and stick to 4 GB.
With prices so low right now there has never been a better time to just get 8GB and futureproof your system. 8GB can help in some situations even with current games. Jack linked you to a great article.
Still it really depends on what you do with your system.
If you don't heavily game or multi task 4-6 is the sweat spot. Because it games well and multi tasks well not super well but well. It also depends on what programs you run do you do rendering or graphics.
More info will get you a better answer.
I would recoment 8 gigs if you really like to game that way it helps if you want to tab out or do more than one thing while gamming.