The volume enhancement you have achieved with Sculptra can last two years, although you may begin to notice a subtle dimunition long before then. There is nothing you can do to accelerate this process.
I would need to have access to your before and after photos to know how much volume you needed before treatment and to see what your face looks like after treatment. For most patients looking to add volume to the entire midface, 4 total vials would not be excessive.
When seeking to augment the cheeks, I prefer to inject 1 to 2 vials per treatment session over 2-3 sessions to make sure you get the best possible result. Sculptra should last around 2 years although there is considerable variability between patients.
Unfortunately, you will have to wait out the slow diminution of the fill over the next years. Some physicians prefer a good pop in volume to show the resuls because a gradual fill sometimes leaves the patient wondering what they paid for, as slowly building results sometimes aren't appreciated until photos are reviewed before and after. We usually will inject 1 to 2 vials and allow the natural buildup to take place with plans to inject a bit more depending on the fill obtained after 4-6 weeks. I feel then the patient can be the guiding decision maker to well matched results and volume.