4 GB shall suffice, but if you buy RAM, buy it somewhere else, as you can get 8 GB RAM (two 4 GB modules, 12 GB total with the already present 4 GB) for less than 50 USD via Newegg or Crucial or other sources, and Apple wants much, much more for less (8 GB total for 200 USD).
You need 1333 MHz 204-pin DDR3 SO-DIMM RAM modules.