If the Implants That Support an All-on-four Fail, What Are the Treatment Options?

If the teeth could not be saved at any cost . A proper diagnosis and treatment  planning is mandatory in such cases. 
In the scientific literature it is mentioned that if a patient smokes  10-12 cigarettes per day, risk of implants failure would increase upto 70%. A thorough tobacco quitting protocol should be followed at least 2 and if possible for 3 weeks prior to implant surgery and  2-3 weeks post surgery. 
Alcohol has a synergistic effect with smoking and interferes with the medications given for implant healing after surgery. So if someone takes good care of All - on 4 prosthesis. Regular annual visits and check ups and strict oral hygiene practices are followed implant will survive with a 90-95 percent survival rate provided patient is keeping tobacco consumption low. Also apart from this there are other medical situations too like high blood pressure or blood sugar which contributes to bone loss around implants and leads to failure. 
Now answering your question even if an implant fails. Doctor needs to see the possibility of grafting at the place where the implant has failed. 
After grafting the site and few months wait , reimplantation can be made possible and prosthesis can be redone. And thus it is important to practice oral hygiene practices diligently specially with implants as they give way easily than natural teeth. 

All on 4 is one of the most successful procedure(99.6 % success rate),  if performed by an experienced team. Its very very unlikely that all implants fail (never seen till today). If an implant fails, we always can add another one depending on the position. At times we need to graft and wait for 4-6 months, before adding another implant. The prosthesis may need to be repeated / modified after adding a new implant.