4 mm thick is very thick for a ring band. Metal benders will likely
break at that metal thickness.
Anneal well and quench hot into pickle.
I assume you do not have a swage block.
I am a Silversmith, we use hammers.
To bend heavy ring shanks, I have a 5 foot long piece of 5" by 2"
piece of hardwood with holes for my mandrels.
I put this in a vice so the end of the wood sits on the floor.
You may have a hole in your bench for this.
Hammer the metal around the mandrel till it is circular. A plastic
hammer will do.
Take ring off mandrel, hammer till it makes a D shape with the ends
on the straight side of the D.
The ends should meet tightly, cut through with a no 4 saw blade.
Solder, quench hot in pickle etc.
Back on the mandrel, hammer till round.
Need more details email me offline.