Predict the order of reactivity of the following four isomers towards ${\text{SN}}^2$  reaction. 
${\text{(I)\hspace{0.17em}CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{Cl}$
${\text{(II)\hspace{0.17em}CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH(Cl)CH}}_{\text{3}}$
$\text{(III)\hspace{0.17em}}{{\text{(CH}}_{\text{3}}\text{)}}_{\text{2}}{\text{CHCH}}_{\text{2}}\text{Cl}$
$\text{(IV)\hspace{0.17em}}{{\text{(CH}}_{\text{3}}\text{)}}_{\text{3}}\text{CCl}$