What is the best way to make my face rounder?

From the photos that you have shared it is quite remarkable the difference in the appearance of your face from what I suspect is a few years ago.  You are still young and I assume that you have had a great deal of weight loss.  Unfortunately, gaining weight does not necessarily insure that it will go directly to your face.  There is no debate that fat injections to the proper compartments of your face will enhance the appearance of your face as well as any filler would.  Finding the fat in you could be a bit of a challenge.  I would suggest some weight gain though I cannot comment on your current status but here is the real message.  Simply because you had 4 Sculptra treatments does not mean that those injections were performed to maximize the effects to your face.  I would suggest seeing a skilled surgeon who is familiar with the deep and superficial fat compartments of your face who can tailor your injections to address those fat compartments that have specifically deflated leaving you looking thin and older than you feel.

Hi Catsadles-
As we age we all lose volume in our face. This process is accelerated in some and can be affected by behavior to some degree. Most often, I will see avid runners have an exaggerated early loss of facial volume which is reflective of overall low body fat. Sometimes regaining weight will help, but we often see that facial volume changes independent of overall body weight. I do think fillers can help, and sculptra is a longer lasting treatment, but does require multiple sessions. Another option would be to consider fat grafting. Good luck!

As we age, we lose volume in our faces due to shrinkage of the fat pads and resorption of bone. If you are an athlete, then facial weight loss can be even more. Sculptra helps stimulate your skin's natural collagen production and works subtly, gradually over time for a more youthful-looking appearance. If you had your injection recently, then I would wait as results can take 1+ years. Weight gain can make the face more plump, but is not a good option because you cannot control how much fat goes to your face and is also unhealthy. 

One option would be autologous fat injections into the face. With this procedure, fat is taken from the belly and deposited back into the face and can create more full appearance. I would research a skilled, cosmetic surgeon who performs this surgery in your area. Good luck!