4 degrees C turns out to be the temperature at which liquid water has the highest density. If you heat it or cool it, it will expand. The expansion of water when you cool it to lower temperatures is unusual, since most liquids contract when they're cooled.
The molecular structure of water is essentially crystalline. The fact that water molecules are less dense when they are frozen is also why ice floats on liquid water. The denser configuration of the liquid water keeps the frozen molecules of the ice from passing through.
Water expands when it freezes because water molecules are not shaped like most other liquids. When water starts getting colder, the molecules move closer together, but the shape of the molecules means that there is a lot of empty space between them. This makes the water take up more space and expand as it freezes
An interesting consequence of this peculiar feature of water is that the temperature of water at the bottom of a lake in the winter is almost always 4 degrees C, since the densest water will settle to the bottom -- if it gets any colder or warmer, it will rise. Ice floats on top of lakes, preventing evaporation (and convection in the frozen layer), and lakes stay liquid underneath, allowing fish and other life to survive.