I really like #4. That's your best shot in the series, in my opinion, for these reasons:
#4 was my favorite of the shoot. That shot actually came about when I was mentally reviewing the criticisms you gave from the last shoot, where I posted a shot that created double chins. Can't have a double chin when you're looking up, right?
Ask yourself these questions about #4 and #6:
- How do you like the lighting and the shadow on the neck?
- Does it look natural?, Why not?
I never even looked for that. Good spot for a reflector or a small second fill?
Do you like the expression in #6? Why not?
Hmm... even though you lead me to the 'why not' question... I actually do like it. I was going for an emotionless pose. What do you see there?
What do you think causes the large hot reflections in the lips of #1,2,3,& 5? What can be done about it?
Two things: freshly applied lip gloss that has the consistency of vasoline and hard light from the umbrellas. Can be rectified by not using lip gloss under lights or maybe using better diffused light.
What is the minimum focal length that should be used in a 1.6 crop camera? Did you follow that rule in #2?
I know you shouldn't go too wide or you start to distort facial features. I don't know what that number is however. That shot is at 40mm, which would equate to 64mm full frame sensor equivalent. Does it look distorted to you?
Robert, thanks a bunch for the criticisms - they are a big help.
First of all she is GORGEOUS!!
Pretty hot, isn't she? 
1. a good portion of her face is cut off.
Point taken. A year ago I would have said the same thing. At the moment I am concentrating more on filling the frame than making sure I get everything inside of the box. A year from now I might go back.
2. No eye contact however im thinking this is what you were going for. But it seems as though the only thing in focus is her right eyebrow.
Yes, I asked her to look down for that shot. I was shooting wide open at that point, so that eyebrow seemed like a logical focus point at the time. A little smaller aperture may have been called for on that shot, especially since there is no eyeball to make pop.
4. I would prefer if more than her eye was in focus
Yeh, f/4 probably would have been better there. Wait until my 85mm 1.8 comes in today... it might get worse before it gets better on this one. I do have to agree with you though. Shooting wide open was a lot easier with my current crappy lighting setup though.
5. Smile looks a little forced. And the eyes are not in focus. It seems as though the nose is in focus.
Definitely missed focus or my hand was shaking for that one. Probably shoulda chunked it. I liked the pose though.
6. My favorite! Face seems to be evenly lit, in focus, and well composed.
I think that one has a DOF more in line with what you are looking for in #4...
A lot of these seem to be a little dark with shadows on either side of the face. These are all just my subjective comments. I could be totally wrong. Great work, and again a beautiful model!
My lighting has a long way to go. I've been shooting for almost 3 years now, and this is my first time trying studio lighting. It's hard! Thanks for the comments.
I personally like #6 best and perhaps thats because I think most of her face is sharp and in focus. #4 second, composition is really good and I love the crop but I think a longer DoF would be better. Nice job.
Comparing the DOF on #6 to the other shots, it'd be hard to argue with you on that. Thanks for the comments!