Not really, Bill. Other than 4C is the temp at which water is the most dense. Slightly less dense as you approach freezing, and considerably less dense as you approach boiling. The reasoning for the maximum density at 4C rests in the unique nature of the hydrogen (polar) bonds of water. These unique bonds are also responsible for salt's ability to melt ice and saltwater's ability to drop below the normal freezing point of 0C (which is why it is so dangerous - water in the antartic freezes skin on contact). The polar bonds are also responsible for water's ability to dissolve salts, and for the gain in density as a result of those dissolutions. But, I suppose I could get into that in another thread.
*I should also note that the "rule of thumb" I posted re: air density is only applicable for temperatures at which humans can survive.
