what i think......well here goes......some advice would be to lessen the pressure on ur pencil when shading the darker areas (use the side of pencil, not tip), im sure u didnt intend for the harsh strokes to be visible (try blending or smudging them). also the first piece seems to be lacking a smooth transition from light to dark. the transition should be smooth unless u r shading areas where hard edges exist (side of nose, ears, ect). the second seems to have turned out better though it seems to need more contrast, another dark value would help. the fourth looks posterized which is prolly what you intended. a general tip would be to quit the habit of outlining when drawing from life or when going for a more realistic style, people who have grown up drawing in a more comic style seem to have a hard time letting that habit go imo which was my case. mind u this is just some friendly advice from one artist to another, i dont mean this in any way to be harsh criticism. you did do a great job in ur initial illustrations the placement and proportion is great. keep up the good work.