I know that water behaves strangely once the temperature drops below 4 (celsius) degrees. In a nutshell, its volume starts to expand (see wiki for further details) below 4 degrees - when temperature reaches zero it freezes. Again, as far as I know and understand, this anomaly has only to do with the density of the water - ice forms when temperature reaches zero.
So my question why in many (all?) cars the temperature display starts to blink (sometimes also give sound warning) when the temperature reaches 4 degrees? Is it just a general warning that it is cold, or does it have to do with some properties of the water?