Macbook Air 13inch, will it last?

Ok, here's the thing. I have an iMac. It has 4gb of RAM and 500gb of storage. I will soon sell it to buy a Macbook Air, 4gb of RAM and 128gb of SSD. But, will it last? I'm a student, I need my laptop to stay with me for at least 4 years. Will the MBA with "only" 4Gb of RAM will do that? My 2009, 3.06Ghz 21.5inch iMac with Mavericks is still killing it but I need a laptop. Can I buy the base MBA with an external USB 3 HD and be sure that in 4 years, 4Gb of RAM will be enough? My budget is limited and I need to make choices... Thanks!