Also check this:
http://i.imgur.com/7VaN0Tc.png
Notice how SSAA is referred in both the horizontal and vertical resolution. 2*2=4, so 2x2SSAA is shortened as 4xSSAA. This can be confirmed by looking below at the combined SSAA and MSAA options. 32xS is 2x2SSAA + 8xMSAA, 2*2=4, 4*8=32. This is the most logical method of labeling the values.