4 year cycles also relate to leap years, the orbit around the sun is almost exactly 6 hours more than 365. on a side note, the chinese lunar calendar more accurately catches those extra days and is only wrong every 10,000 years or something, whereas the modern gregorian calendar based time/date system actually needs a bump of a second every other year, i forget how frequently, but it's enough that the clocks will be an hour wrong in less than a century.
i forget the details. it's something that terence mckenna talks about in 'invisible landscape' - that was where i first learned about it.
