I believe that OSX runs best on 8GB even with an SSD installed. I got rid of my 2011 MBA for a 2013 MBA mostly because of the lack of RAM. An SSD will reduce the impact of a lack of RAM, but in my opinion it doesn't replace the need for adequate RAM. I'd recommend 8GB of either 1333mhz or 1600mhz (neither is going to be noticable, but if you can get 1600mhz for only a little more you might as well) or if you can swing it go for 16GB (although probably overkill to be honest for most).