Callan draws to a close with a reminder of what tue series was always all about from the very beginning with the Armchair Theatre teleplay. Callan was a loose cannon of an agent and his emotions were known to guide his actions in the job. The finale underscores that perfectly. It wasn't going to be a pitch battle of good guys versus bad guys with the bad guys suffering ultimate defeat. How often do we see that in this genre anyway?
The series ends with a proper finale and so does the 4th season which opens with Callan captured in an East German prison. He is heavily drugged and subjected to psychological torture. In the end Hunter's superior decides Callan is worth trading for but not Hunter who already arranged Callan's funeral and burial in his absence. The trade was for Richmond who is the subject of the last three episodes. Seeing the first episode of the season explains the motivations better in the last episode. The impact may be a little diminished if one were to forget or miss the beginning of the season.
Another reviewer mentions Callan shooting out the bulb in the finale which is the same type of image used for the credit sequence. I don't think there is any relation at all except that Callan is capable of shooting the bulb. Mitchell likely just added the scene because he could.
A great episode. It ends with things basically as they were when it started. Callan hating his boss for the rotten jobs he is given to do with no reward for the work except a paycheck and continued freedom such as that is since his employers already threatened to kill or imprison him. His deep resentment for being hung out to dry whenever it suits his superiors shines the brightest.