Mid 2012 Macbook Air - upgrade RAM from 4Gb?
Nope, the RAM on the MacBook Air is soldered in so you won't be able to upgrade it without breaking the computer. You need to make sure the amount of RAM is enough when you buy the machine.

If you don't mind you can downgrade the OS (the lowest you can get on 2012 MBA is 10.7.4), that will probably reduce the amount of RAM needed.