Predict the order of reactivity of the following four isomers towards S_N² reaction. 
$\left(\mathrm{I}\right){\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Cl}$

$\left(\mathrm{II}\right){\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}\left(\mathrm{Cl}\right){\mathrm{CH}}_3$

$\left(\mathrm{III}\right){\left({\mathrm{CH}}_3\right)}_2{\mathrm{CHCH}}_2\mathrm{Cl}$

$\left(\mathrm{IV}\right){\left({\mathrm{CH}}_3\right)}_3\mathrm{CCl}$