All on 4 is a marketing term used by implant companies for a treatment protocol whereby two of the back implants are angulated to avoid certain structures. If you have four implants which are properly spaced in the upper and the lower Archs and you do not require the Denture flange to hold your lips in the proper position, there is no reason why you cannot upgrade it to a fixed bridge alternative.

The expert in this field is called a prosthodontist. Just look one up in your area.