They don't need fill, what they need is a key light. There's no directionality to the light on the faces. It looks like you're afraid of the light, so you're tucking them in shadow to avoid harsh shadows and highlights.
They also look noisy. Maybe they do need fill after all, but you lifted the faces out of shadowy darkness?
I like the backgrounds and the poses, mostly. #2 is looking out of the frame and leaning back, which isn't creating a very balanced and pleasing frame. The rest look good, though.
1. As mentioned, the image needs fill light, 'though the background lighting is very nicely rendered. It also needs to be leveled. Try and avoid placing the horizon at/near the centre of the image, and don't clip bits of limbs, such as her left hand.
2. This would have been a great shot with a little bit of fill light. Watch the minor details. Her shirt is partly tucked into her trousers, and try to avoid rotating the subjects head so far as to obscure the sclera totally on one side.
3. Shooting from above is not usually a good choice for this this style of image, and this case is no exception. Looking down on her like this puts her in a subservient position. Also again, details; a quick brush of the hair would have helped a LOT.
4. The colour seems off here; a bit high on the green channel. I think with a bit more exposure this would have been a nice shot.
Thank you for the feedback amolitor. You pointed out how noisy the pictures were and you are certainly right. This was my first real attempt at photography and had no clue about proper lighting (and after 6 months still trying to learn that) and I did another big no no...I took these photos with a kit lens and then cropped them down in Photoshop. I know I eventually need better glass, but have to work with what I have for now which means I need to get closer to my subject to avoid cropping them down so much.
I now see what you are saying about #2.
Anyone have suggestions on good books / websites that can assist with leaning about proper lighting?