Besides differences in pricing there are also differences in materials.  One of the most common materials used for all-on-four and all-on-six dental implants is full zirconia.  It is a very hard material so it won't stain or break easily.  When shopping around please be sure to ask what material your final set of teeth will be made from.  Many offices only provide an acrylic/resin overlay with a metal bar as the final and some offices that provide the resin overlay with metal bar use a resin that is not meant to be a "final" material and yet market it as such.  There are also offices that charge another fee for your finals without fully explaining that the set you received after surgery is only temporary.  Please do your research and fully understand what exactly is included in the fee you pay because some even charge separate fees for the surgery portion of the procedure.  Your provider should be transparent and answer any questions you may have.