The Prisoner od War Escort Company was proably part of the WW-I army of occupation.
Your G G grandfather should have received a WW-I Victory medal since he served during the prescribed time frame for it. That medal, which you can view online has several battleclasps available - one for each of the battles, one for each of the overseas countries if the recipient was not involved in any fighting, and a defensive sector clasp for the early training in Europe. See the webpage for The Institute Of Heraldry listed below
Since you do not have his actual discharge paper it is probably impossible to trace his activities in Europe. The fact that he was photographed wearing a 4th Inf Regt disk is probably sufficient - Absent any further documentation which you probably will not find except if there are records for the 4th Inf Regt at USMHI (AHEC) Carlisle PA
Probably the best online site for inforation about the 4th Inf Regt is: