I believe hot water is running through the slab, not through insulated, interior walls. That will cool the water quickly, although 4 minutes seems a bit long to me. Out master bath is at the farthest point from the water heater possible, and, while it takes a while, I don't think it takes as much as that. I even notice that when I take a shower a couple of minutes after my wife gets out, I still have a wait for hot water, so the water cools off in the pipes fairly quickly.
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