Why Is Zirconia the preferred choice for All-on-4 fixed teeth? 

yes that is totally ok.... the implant have to be correctly positioned and full integrated. Usually with 4 implants we aim for 10 teeth and if there is adequate bone we prefer to place 6 implants. It would be very specific to the particular patient and the amount of bone available, the opposing teeth, the length and strength of the implants. We have many patients with 12 teeth on 4 implants and they are working well.