The first photography has direct eye contact with the camera. This gives a very personal feeling to the photograph, and combined with the very warm smile in this image, gives a very friendly, welcoming feel to the photograph
This next picture had emma looking up and slightly away from the camera. In my opinion I do not think this photograph works as well because it is slightly unflattering, with a lot of the neck on show, and the eyes are slightly less prominant in the picture, removing the personal connection.
Now side on is far more flattering, still looking slightly up, but the facial features are well presented, with the nose not breaking the cheek line. It gives a very classical feel to the image as well, and is less personal. It gives more of a feeling of capturing a moment in time rather than a posed portrait as well
Probably my favourite of the four photos, this one is looking to the side, but not up, instead looking straight. It again gives that classic, less personal feel to the picture, and the facial features are flattened slightly by the angle. I personally like a mix of images of straight on, looking at the camera, and turned away, looking away from the camera. Both have different feels, and both are images that work. Looking away from the camera needs to be controlled more carefully, for example the nose not breaking the cheekline, as this can look rather odd, but both posed right are portraits that I like.