Sorry to disagree, I hope I'm not too late, but I don't recommend getting an MBA with just 4 GB, and I'm not speaking just because of running Parallels, but in general for future proofing it. First because you'll not be able to upgrade it later, the RAM is soldered so the RAM you get out of the factory is all it will ever have, secondly, 8 GB costs more but it's just about 10% of the overall price, thirdly, on the OS X side, running a browser with medium to many tabs open will easily exhaust the 2 GB left (Windows is occupying the other 2 GB).
Finally, and this is very important, the onboard Intel HD 6000 graphics have a VRAM of 1.5 GB but it's shared memory, meaning it uses the RAM, so, out of those 4 GB you have to subtract whatever the GPU is requiring at the moment.
The system requirements for Parallels state 4 GB RAM as minimum but recommend 8 GB and that's not even taking into account GPU shared memory: http://kb.parallels.com/en/123276
In summary, considering shared memory, the inability to upgrade the RAM later and the fact that it doesn't cost much (relatively speaking). Get the 8 GB, that's my recommendation.
Last edited: Jan 31, 2016