I think your model is spot on perfect. She is well defined, looks slightly relaxed, she is very cute. You exposed very well for the girl.
I think maybe a slightly different background would make this stronger. The style of background is fine (she stands out well from it, it's a nice mesh of lines and textures without detracting from the model). I do think the lighting is bright on the background. It overwhelms a little where the sun is shining off of the yellow blades. I hope this makes sense and is helpful (it's not intented to be harsh).
Very nice detail and skin tones on the subject. I'm not sure that the vignetting effect is real effective here. I think perhaps a wider aperture to shallow up the DOF might have worked a little more effectively. Nonetheless, this is a nice portrait and much better, imo, than the rating given by the three people who are either too unmotivated or perhaps too timid to explain why this has a rating of "4". As for me, I think its much stronger than that. I hope you find my comment helpful.
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