For people who want to change out the rear aperture in their A4 carry handle, I have outlined here how to properly do so. Some people like to replace the rear aperture with a tritium night sight for low light shooting conditions. In these instructions I have simply taken apart and reinstalled the stock aperture, but the directions are the same when reinstalling a tritium night sight.

Make sure weapon is unloaded. Wear safety glasses.

Remove carry handle from upper receiver. With a small hammer and a 1/16″ punch, drive spring pin out about 3/8″.

Pull out the punch while holding the knob and slowly pull the knob off being careful not to lose the ball bearing and spring in the knob. It’s best to hold the assembly so the knob is facing down and pull the carry handle away from the knob. Then gravity will keep the spring from falling out of the knob. The ball bearing itself may be stuck to the base with oil. Place ball bearing on top of spring and set the knob aside.

Unscrew the aperture screw and pull it out of the aperture and sight base.
You may now remove the rest of the parts.

Reassembly
Install the flat spring as shown into rear sight base.

Place aperture on top of flat spring as shown and push aperture screw in through the base and the aperture.

With a plastic mallet tap the top of the aperture while pushing on the screw until it enters the hole in the other side of the base.

Screw aperture screw in lining up the index mark on the aperture with the middle index mark on the base as shown.

Place windage knob back on the aperture screw. Push the knob on lining up the spring pin hole with the hole in the screw. Push 1/16″ punch back through the hole in the knob and the screw to hold the assembly together.