If you want to upgrade your Mac when you buy it from Apple it's very easy to do so.
After you've selected the model you wish to buy you'll be presented with a screen that asks you to configure your Mac.
When we checked the memory upgrade was top of the list.
For everyday tasks 4GB is perfectly fine, as internet browsing, running office software, and light gaming will work without issue.
You can of course use the built to order feature on the Apple website to upgrade the RAM at point of purchase, and we recommend in some cases that you do this, as it may well extend the productive life of your Mac.As a general rule we would suggest that you have at least 8GB of RAM fitted in your Mac.These are the 27" i Mac (either the Retina or non-Retina models), the Mac Pro, and the non-Retina 13" Mac Book Pro.For all other Macs Apple has chosen to solder the RAM, making it virtually impossible to remove.
Scroll down to the Memory section and tick the box you want.
Recently Apple has started to offer commonly requested configurations at the top of the page in the Get It Faster section.