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

Dental implants help stimulate bone and distribute forces. Patients that wear dentures have atrophy as a cause of movement and loss of chewing pressure.

All on four prostheses will avoid bone loss because the dentures are not depending on the gums and underlying bone for support. The dental implants support the prostheses and all of the forces are transmitted through the implants into the bone in a similar way that forces are transmitted to natural teeth.  Dentures without implants cause bone loss because they put pressure on the gum tissue and bone from the top down. None of the forces are transmitted into the bone which actually stimulates the bone and maintains it. It's a difficult concept to grasp but very well proven.

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

The tension created by implant placement will maintain bone volume, but where teeth/implants are missing, bone can shrink.  The more implants place, the more bone preserved.  8 is better than 4.Facial structure preservation is usually determined by the restoration itself, not the bone/implants.  The soft tissue support from the restoration is what gives most of the volume.