You're right about how density works, but I don't have it backwards:

A given mass of water:

- At 20C is less dense/higher volume than water at 4C

- As you cool it the density increases and the water shrinks

- At 4C the water is now at maximum density and minimum volume

- As you continue cooling it from 4C, it starts expanding again

- At 3C the water is lower density than it was at 4C and is again expanding

- At 0C where the water starts to transition to a solid the volume expands significantly

I guess I wasn't clear that the expansion only starts again as you go from 4C -> 0C in the liquid phase.

ah OK, I get your point :) It doesn't really matter though because water when liquid doesn't cause any problem when expanding or contracting, its level in a vessel will just slightly change. It's only the ice that can fuck up pipes etc