You can always upgrade the storage to a bigger unit once you have the money, but you are not able to ever upgrade the RAM, thus I say, get the 8 GB model.
You can always upgrade the storage to a bigger unit once you have the money, but you are not able to ever upgrade the RAM, thus I say, get the 8 GB model.
+1
Can you get by (for the use described) with 4 MB RAM? Yes.
There's no practical way to augment RAM on a MBA.
There are ways to increase memory/SSD capacity, internally or externally. For instance, USB 3.0 Flash Drives are cheap and pretty zippy for many uses, if you need extra memory in a pinch.
If you update to the latest MBA models frequently, you can have competence performance and then trade up at the next release.
But if you keep your Mac longer, the 8 MB RAM improves future proofing. You'll get better performance for intensive apps. Software continues to get more RAM hungry.
Look at the performance in the MBA's in this test. It's not a perfect comparison, since the 4 GB MBA is 2011 & the 8 GB is 2012. But it may give you some idea.