The poses used in the first three pictures are pulled off really well.
It also shows that you're ambitious, and you're not one of those people who can only draw things from one angle or in one specific position.
It shows a great deal of spacial comprehension.
The anatomy is also pretty good in the first three. The second picture probably has the best anatomy.
As for picture number four, I can't tell why there's such a change in quality/style of art.
There's a significant change in the level of ambitiousness and complexity when it comes to the fourth picture. I would guess it's probably number four is digital art, whereas the first three pictures are scanned? (It is digital, isn't it?)
It can be hard to control a mouse (or even a tablet) at first, but it gets easier with practice.