This is the vacuum modulator, which either screws in or is retained with a clamp depending the C4's vintage. To adjust the vacuum modulator, which controls shift points, remove the vacuum hose and use a small common screwdriver to make adjustments clockwise or counterclockwise, but never more than one-quarter turn at a time. Never go more than four turns in either direction. Clockwise, you increase diaphragm spring pressure, which delays upshifts and makes them firm. Counterclockwise makes shifts softer and earlier. Check the vacuum hose for leaks, which can cause both engine and transmission performance issues.