Mavericks is extremely memory efficient. 4GB is more than enough. If you really needed 8GB of memory you wouldn't need to even ask, you would just get it.
I'm really interested in trying it out to see, but I just sold my MBP for the MBA.
I had a MBA around a year ago with only 4gb. I would do very basic tasks, and page out occasionally. I always thought OSX was very bad with memory usage - windows was always fine with 4gb aside from gaming.
Thanks for the help guys. I think I'll be going with 8gb as it probably will take over as my main computer down the road. I also haven't seen 8gb refurbs (unless high end models) on the Canadian refurb store ever.