To replace a full set of 14 teeth, you need support at the back where you chew. With All-on-4, implants group at the front of your jaw. So, unlike All-on-6, adding back teeth is like a building a balcony with no support underneath.
If you chew hard on the back teeth, the leverage forces can snap the bridge or the implants.
All-on-4 failureIf just one implant fails, the whole system fails. You have to remove the bridge, do new surgery, wear a temporary for months, and pay for everything again. You're back to square one.
Implants are successful 95–98% of the time. But if you are in the unlucky 5% — maybe due to smoking or bad luck — the difference between 4 and 6 is huge.