$P=\frac{Q}{t}\xrightarrow{Q=mc\Delta \theta }P=\frac{mc\Delta \theta }{t}\Rightarrow 40=\frac{\left| 0/4\times c\times (40-70) \right|}{5\times 60}\Rightarrow c=1000\frac{J}{k{{g}^{\circ }}C}\xrightarrow{\div {{10}^{3}}}c=1\frac{kJ}{k{{g}^{\circ }}C}$