Predict the order of reactivity of the following compounds in $S{N}^2$ reaction:

I) $C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}Br$

II) ${\left(C{H}_3\right)}_{2}CHC{H}_{2}Br$

III) $C{H}_{3}C{H}_{2}CH\left(Br\right)C{H}_3$

IV) ${\left(C{H}_3\right)}_{3}CBr$

The reactivity in $S{N}^2$ reactions follows the reverse order as the steric hinderance around the electrophilic carbon increases in that order.

I > II > III > IV