I think it's entirely possible people can learn to do an everyday task wrong. People learn bad eating, bad breathing, bad sleeping, etc. Beyond that, the whole shitting-in-water, sewage system only makes sense to me in a large-scale, urban context. Nature indicates people were meant to be shitting in holes in the woods, or with an outhouse.
"The Bristol Stool Form Scale or Bristol Stool Chart is a seven group classification of the form that appears upon defecation of human feces. It was developed by Dr. K. Hering at the University of Bristol and was first published in the British Medical Journal in 1990. The form of the stool depends on the time it spends in the colon. Although not an exact science, the scale is used to help a physician recognize stool patterns for the purpose of diagnosiing digestive conditions."
P.S. Their version of the chart has a wider range of colours :-)
Upon reflection rocks, you're right. A four does appear more streamlined, less fuss & muss. I think a four would bring a smile to my face unlike the horrific, panicky feeling I'd get after a 7 or a 1. I've had my share of sevens, but thankfully no ones.