How will All-on-4 implants avoids bone loss?

Depending on the age of the patient as well as the anatomy available for implant placement, the first priority would be to place six implants on the upper arch for an All-on-6 and then All-on-4 for the lower arch. It is true that the areas where there are no implants can continue to experience atrophy or bone loss. However, the fact that the prosthesis is a hybrid that contains acrylic lends itself to filling the voids that would otherwise cause facial collapse. 

In regards to your question. The 4 implants should be well distributed and connected by a well fitting prosthesis(Teeth)

With this in mind as you function the teeth transmit compressive and tensile forces to the bone. As bone is stimulated it maintains its structure(Wolfes Law), liken it to going to the gym and working out, if you stop working out and stop challenging your body your muscular strength will diminish.

Whilst implants are they they stimulate the bone