Ops, I missed it. Thanks for letting me know that I was wrong. Let's put our AIs skills to work here now!

From what you've shared, it appears that each sequence of four letters progresses in alphabetical order, and therefore the man has likely been receiving similar letter sequences each month. One possibility could be that he is a member of a subscription service such as a book club or movie rental service where he receives new selections each month.

Another possibility could be a monthly employer bonus given for work they do or run. Based on the information provided in the prompt though there are some limitations to what we may come up as it lacks backgound info.

Did any of those guesses make you laugh at least? :)