The second needs more light on the eyes too. She looks like she's going to fall backward out of the photo. Just an awkward position or too tight a crop.
Third on is nice, maybe a little underexposed, but the angle of the fence in the background doesn't work for me.
4th one is probably strongest, but it needs some light above them to keep their hair from underexposing.
I agree with what was said already. I also think the facial expression in #3 kills it. A smile would have been better for this particular model I think because she looks like she is unhappy with just the normal un emotional face.