The expansion is actually more than double that, 9% is the figure I've commonly seen. See for instance Wikipedia [1]: An unusual property of water is that its

The expansion is actually more than double that, 9% is the figure I've commonly seen. See for instance Wikipedia [1]: An unusual property of water is that its solid form—ice frozen at atmospheric pressure—is approximately 8.3% less dense than its liquid form; this is equivalent to a volumetric expansion of 9%. The "4" might be coming from the temperature at which water's density is the largest, which happens at 4 degrees Celsius. Water is weird. :) [1] https://en.wikipedia.org/wiki/Ice#Physical_properties https://en.wikipedia.org/wiki/Ice#Physical_properties