4" stainless ferulle brazed with a special flux coated 45% pure silver solder to a copper slip joiner. These fittings have been requested by the home builders to allow easy joining of copper to tricalmp fittings.
Use one of these, slip a 4" copper pipe in, true it up and either braze or soft solder your pipe in to fit instantly to a keg or any other 4" triclamp fitting.