Why do water expand on cooling below 4 C?
Essentially because the water molecules get close enough (with the increase in density) that they start repelling each other (because they have a slight charge disparity), and forms into a hexagonal structure of least energy as it cools further towards 0 degrees Celsius. As the vibrational energy (due to heat) is lessened the molecules can form structured *stable* hyrdogen bonds allowing a formation of hexagonal structures. A detailed picture of this structure can be found here: http://en.wikipedia.org/wiki/File:Cryst_… This is mainly due to the fact that oxygen attracts the electrons in the bond (between O and the 2 Hs) closer to itself, causing the H ends of the molecule to be slightly positive and the O end to be slightly negative, in terms of charge.
