Copy the videos below, locations at the top of the event graph.

Next go to the BP_FirstPersonArms_Anim and create a new variable called Last Sight, set its variable type to BP_MasterAttachment. Then copy the images below adding the highlighted nodes.

Part 1
Part 2
Part 3

Next go to the BP_M4A4ReddotAttach then to its class defaults and in the ADS Camera Transform set the Value (Transform) like the image below.