CREATE TABLE table_71891 ("Res." TEXT, "Record" TEXT, "Opponent" TEXT, "Method" TEXT, "Event" TEXT, "Round" FLOAT, "Time" TEXT, "Location" TEXT)

Who was the opponent at GCM D.O.G. 4?
SELECT "Opponent" FROM table_71891 WHERE "Event" = 'gcm – d.o.g. 4'
SELECT "Opponent" FROM "table_71891" WHERE "Event" = 'gcm – d.o.g. 4'