I have absolutely no doubt that a MBA will serve you well for 4-5 years, as long as you don't expect it to play the fanciest games available 5 years from now.
I have never kept an Apple laptop for *less* than 5 years, and I see no reason that my MBA will be any different. The only thing that "breaks" is the battery, but you're going to have that problem on any laptop.
Actually, now that I think about it, the battery on an Air might actually last you longer. Since the battery holds a charge for so long (I've often come back to my laptop after a week of not using it, and the battery is still almost full) I don't charge it nearly as much as the 15" MBP it replaced.