Hi Sara!
The 4 point mannequin assignment is very tricky. I agree that curving the lines would help to keep the roundness on the volumes of the body. As for the head, it might be beneficial to review proportions of the human body. I tried to show my thinking in some images below.
I'm not a professional, so I'm just giving advice as best I can. But as a general guide, female shoulders are usually 2 heads wide (2.5 heads for males). So if you are using the rectangle as a guide for the width of the shoulders (as in your image above), it might be useful to use it as a guide for the size of the head. Also, because of the angle from which we are viewing the head (like a fisheye lens), we are likely going to see more of the underside of the chin than the front of the face. It should kinda look like the head is tilted back some.
I did 3 drawovers of your image to try to illustrate my thinking.
First is to identify the shoulder width proportions and their centerline, then fit 2 heads within that space (1 head in the center and .5 of a head to each side for a total of 2 head witdths):
Second is to warp those guides with the curving perspective. Remember that the curvature gets more extreme the further away we are from the horizon line:
And third is to fit the body to the new proportions while trying to keep in mind that, from our point of view, we are looking up toward the underside of the head:
Again, I'm an amateur so this may not be totally correct, but this is how I would go about trying to solve the head proportion issue.
I hope this helps! Cheers!