Well, well - though I've dropped the original blog which covered tetrads "per se", I give a link-back to this well-thought gentleman.
In this current blog, tetrads have been updated to "four-way thunks", but I do have a Lulu book which contains the original doctorate thesis. (The former blog title has been claimed by someone who titled it "womanonline". I'm sure that "tetradsystem" means something completely else...)
The Tetrast: What of these other fours?: "“Why tetrastic?,” some fourfolds echo each other in ways for which I have not yet managed, at least to my satisfaction, to uncover the reasons, even when the fourfolds separately from each other have seemed clear enough. Turn a sign this way, then that, align it with others, the world seems to crack open, and the chase may be on."
This quote sums up my thoughts about tetrads/four-way's - they challenge thought and transport it into new dimensions.