According to the heat equation:
Q = m × c × ΔT = m × c × (T2 - T1)
where Q - heat, m - mass, c - heat capacity, T2 - final temperature, T1 - initial temperature.
c = Q / [m × (T2 - T1)]
As a result, the heat capacity of the water is:
c = 66.90 kJ / [1.00 kg × (20 °C - 4 °C)] = 4.18 kJ/kg °C = 4.18 J/g °C
Answer: 4.18 J/g