How do I add macbook air memory - Apple MacBook Air

In short, you can't. This is why it is even more crucial to consider your choice at the time of purchase. Air is created to be as small as possible sacrificing upgrading capabilities like chip and RAM sockets to have them soldered to the board. You may be lucky and have a model where the internal screw holes match another model's and be able to upgrade that way, but it isn't going to be cheap because you have to buy a whole new logic board and that's gonna be at least $200-300 out of pocket, and that's if you're able to sell your old one, most likely you're going to have to pay anywhere from $400-700 for the different board and the top end price is getting close to the cost of a new unit.

Http://www.everymac.com/systems/apple/macbook-air/macbook-air-faq/macbook-air-processor-ram-limitations-superdrive-capabilities.html

Macbook Air has a different memory set up. The memory is soldered on to the logic board when the unit is built. As far as I know there are no direct memory upgrades developed yet for the MacBook Air.
About your only option is to replace the logic board with one with 8 GB