DefInately go with 8 as it can't be upgraded later on.
4/5 years is an ambitious target to keep a laptop primarily known for its thin design, not for its powerhouse cpu. it will need all the help it can get. And as noted here, once you buy you can not upgrade the RAM.
Get the 8.
I'm shocked people are even considering 4gb to be enough for the next 3-4 years!
NO, it will NOT be enough! Go with 8, especially because you are using integrated graphics which shares with RAM instead of having its own discrete RAM.
I'm just going to say, a Macbook Air/Pro is not the kind of laptop you want to keep for 3-4 years. They are already behind in technology for the price (minus the Macbook Pro Retina) and they update every year.
Ok thanks guys, I'm gonna go with the 8go of RAM, I don't know if I will keep it that long but, just in case I couldn't get "enough" money to buy the latest model in 2/3 years..you know
Plus when I'll get ride of it I guess more people would be interested in 8go than 4go
Again thanks