great practice, if you don't mind me pointing some things out:
- on the first one the septum is too thick at the bottom so there should be less space in between each nostril,
- on the second one, there is less space between the chin and the mouth and the neck is also a little too thick on yours as well
- you could have pushed the angle of the face more on the third one as well it is more 3/4s than what you have, but very nice rendering on the head either way
- and for the fourth one the nose comes out farther and is generally bigger as well
You probably already saw most of these, so it may not help much but good practice and keep up the good work!