The volume of an ideal gas ($\gamma = 1.5$) is changed adiabatically from 5 litres to 4 litres. The ratio of initial pressure to final pressure is:\\
(1) $\frac { 16 } { 25 }$\\
(2) $\frac { 4 } { 5 }$\\
(3) $\frac { 8 } { 5 \sqrt { 5 } }$\\
(4) $\frac { 2 } { \sqrt { 5 } }$