They are all really good I think and I don't have much expertise with portraits, so here i just an idea. In the first three drawing the lines are obvious and if you use them very concious they can be a great style effect. But i think right now you do not control too much where which line shows how clearly. For example in the second drawing one ear is really clearly lined out the other has no lines at all. And it migth be interesting to try to form all the edges without actual lines as a practice. I think that would take more time though. But I really like the effect, how you use limited lines. The fourth one is different obviously, I am only referring to the frist three. But the style of this last one is also very appealing!