The cost of an all on 4 dental implant includes:

• the 4 dental implants
• their 4 prosthetic abutments to maintain the 12 above dental crowns 
• the 12 temporary crowns above the 4 implants
• the 12 permanent crowns that will replace the 12 temporary crowns

I might need to add to the price the options:

• the dentist fees and an x-ray
• the teeth and root canal extractions

The price variations to this price are:

• the type and brand of the implant: each dentist offers his favourite dental implant brand, just ask if you need another one
• the technique used and any additional procedures: if you would need to add a dental bone graft or choose an all on 6 dental implant  
• the material chosen for the crown: it is often between fused to metal ceramic dental crown or zirconium dental crowns and in your case the first type would help to have a cheaper all on 4
• if you prefer an "immediate loading" protocol meaning all shall be donw in 1 trip instead of 2: in this case the cost and time taken would be very different

The "All on 4" dental implant cheap price is an "all on 4" dental implant system optimized with a package offer. The protocol is the same as a standard all on four but it includes a package with accomodation in some cases. Dental implants are in titanium and dental crowns have to be double checked in each case but most of the time if the dental clinic has its own dental laboratory the dental crowns are in zirconium. The package means extractions, consultations, temporary crowns and x-rays are included.