SN2 reactivity and steric hindrance

2022 Phase 2 1 question

1. Predict the order of reactivity of the following four isomers towards $S_N2$ reaction. (I) $CH_3CH_2CH_2CH_2Cl$ (II) $CH_3CH_2CH(Cl)CH_3$ (III) $(CH_3)_2CHCH_2Cl$ (IV) $(CH_3)_3CCl$