Suggested hardware requirements for Xcode 7
This question is primarily opinion-based, but in my experience, your MacBook Air should be good enough for you to use to begin iOS programming. I currently use a 15-inch Mid-2009 MacBook Pro (2.66GHz Intel Core 2 Duo, 4 GB RAM, 256 GB Intel 530 Series SSD) for my iOS app development and it's alright. Would I like a newer MacBook? Of course. But, it's not like I'm making a complex 3D-platformer using Metal or something, I'm just developing a simple 2D game. Apple does a good job of supporting older hardware, so it may be a few years before I get a new Mac. In short, yes, you should be able to use your MacBook Air for learning how to do iOS app development, just don't expect to have an easy time developing complex 3D games on it. The fact that this machine is lean on RAM is compensated for by it having an SSD. This will greatly help with the memory intensity of running a dev env including a simulator.