I'd go with 5. I did 4 the first time around and just regretted not having a fifth (especially a tank). I hit 70 and running around dotting people up in AV turned into a gimmick real fast and I realized I couldn't do instances.

Don't let your current hardware limit your choice, you can always upgrade. And don't underestimate it - I started quad boxing on my laptop with 2GB ram and just an nvidia 7900.. ended up being on the road for 3 months and haven't had a chance to get home and get a proper desktop. I'm currently 5 boxing on the same laptop - graphics are low except for max spell effects and the main window gets great fps, only things that really hurt are Shatt, Org, and AV. Everything else is great and I took it all the way to 70.