i wouldnt buy 4GB in todays world unless it was all i could afford.
my quick example is from my Mac Mini but makes sense and works for this example. the computer would generally be using 3GB of ram with my daily activities and when i upgraded it to 8GB it then would run at around 5GB of ram with the exact same daily use.
so would it run and function fine with 4GB yes BUT as you can see when given 8GB it used it and so less constant management of having to live with 4GB.
i honestly dont even understand why they all dont have at least 6GB today considering my phone has 3GB in it
I have the same kind of experience. Never really seen any slow down. My opinion is that if you have an SSD then for general use there really isn't a need for 8gb, in that most users will not see any issues. However if you do lots of intensive tasks or virtualization etc then chances are its a work machine and should be treated as an investment and go for 8gb. If my job requires a computer, no way I'd skimp on it. 2gb vs 4gb such as on the S3 is one thing, but there should be no need to feel apprehensive about a machine with 4gb for general use.