Personally I have a system with 8 and 16 gigabytes and neither has EVER run out of memory. 4 was kind of borderline under certain uses.
They both have small 1.5 gigabyte ram drive for my web browser/cache(but saving/loading on startup/shutdown does take more time, so for some systems, this is a bad trade off. With one system on all the time, it works out very well.).
For the price, I would generally get 16 gigabytes just because it does not cost much more and things take more and more memory as time passes(new programs are NOT getting more efficient on memory usage).