I'm also 5' -- I am most consistently a size 2P in most petite brands (AT, BR). However, I am all over the map with Theory. I have a blazer that is a size 0. I have two dresses that are size 2, but I have tried on other dresses in that size that didn't fit at all (could not zip), and sizing up made the length all wrong.
I don't own any of the pants as I find the rise too low, but if it wasn't for that, I'd take a 0. They do fit quite narrow in the hips and thighs, which is what I need, as that's where I am smallest. If you are an inverted triangle (sorry I forget what shape you are) then I would start with the size 0. If you are more average below the waist, go with the 2. If you are curvier, try the 4.
I do like the pants and I would experiment more with Theory except what drives me nuts is the lack of lining. The fabrics are very nice, but scratchy without a lining. And if I'm going to pay that price... I expect a lining! What is up with that???