$(2)$ (fig.)

$(3)$ $\mathop {C{H_2}}\limits^ \bullet   - CH{(C{H_3})_2}$

$(4)$ $\mathop {C{H_2}}\limits^ \bullet   - C{H_3}$

The stability of free radicals is mostly affected by the factors like resonance, hyperconjugation and inductive effect. In compound $ (II)$, there is high resonance effect due to which it is highly stable than other three compounds. The effect of hyperconjugation is directly proportional to the number alpha hydrogen. So, compound $I$ has six alpha hydrogen atoms, compound $III $ has one alpha hydrogen atom and compound $IV$  has three alpha hydrogen atoms. Therefore, the order is $II > I > IV > III.$