My first thought about #1 and #4 was "SHE'S GOT NO NIPS!!!". That bothers me, but other than that, not a bad first session. Here's my take on these....
#1 NIce face, but other than the lack of nips, I can't make out exactly how her body is laying. Some will see this as mystery~ others will just think it weird.
#2 She looks a bit too trashy for me, but there will undoubtedly be some who disagree with me (such is the beautiful thing about art!). At the least, I would touch out tan lines and the tag in the teddy.
#3 Good lighting. Nice pose, tan lines need attention, too.
#4 Nice expression, but the light hitting her elbow draws my eye away from her face. Oh and the no nip thing (have I mentioned that??

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#5 There's a reason Shane likes this one, I think. The lines are good and the light is better. We can now see the orientation of the body...sorry her body. It's easy to slip into an objectification mode when critiquing pictures. (Something I think we need to be aware of, but that's a different topic.) The only thing I think that would've improved this one is the old addage of "expose for the highlights, develop for the shadows".
An excellent article on Luminous Landscape can be seen here. Thanks to Michael Reichmann for all his work on this site, it's an awesome source of info!
These are hard for most folks, but after a while it becomes second nature to be in the room with a nekkid person. Then you can focus on the photographic end of things.
Make no mistake, I think you did a really good job on this first try and I think with a bit of practice, you'll rock these!!
Hope this is helpful!
Ginger