I don't place too much emphasis on this idea, but I started thinking about it in 2020. If you recall, there was a great deal of discussion about The Science, and although few people noticed, it was the 400th anniversary of the book that launched modern institutional science, Francis Bacon's Novum Organum. It also happened to be the 400th anniversary of the Mayflower voyage, and the Mayflower Compact was the founding document of democracy in the New World. This was a remarkable coincidence, democracy and science both summarily discarded exactly 400 years after they began.
So I started looking into it, and do you know, curious reader, that our calendar is actually a 400-year calendar? That's how long it takes to go through one leap-year cycle. We have perfect eclipses because the sun is 400 times the size and 400 times the distance of the moon. The Greek Dark Ages lasted roughly four centuries, from around 1200 to 800 BC; similarly, 400 years separates the fall of Rome and the reunification of Europe under Charlemagne in 800 AD.
400-year periods figure into Biblical history as well. The Hebrews are said to have been slaves in Egypt 400 years. 400 years is said to be the "Intertestamental Period," the time between Malachi and the arrival of John the Baptist. The death of Socrates in 399 BC is also noticeable, in that Socrates, like Christ, was executed for his ethical teaching. Interestingly, 400 divided by six is 66.666... which provides occasion to remark that the number of the beast refers to 2/3, mind and body without heart.
At present, it was 400 years ago that the Shakespeare First Folio was lumbering forth into the world, and Hamlet is coming back for his revenge.