The correct order of reactivity of following compounds in SN1 reaction is:

Explanation

I : most stable. From the double bonds, we can see that the strcture is stabilized due to stable delocalization and distribution of pi electrons. II : least stable. It exhibits anti aromaticity. Anti-aromaticity is a concept in organic chemistry that describes a special case of aromaticity in which a molecule or a cyclic system possesses a conjugated π electron system but fails to exhibit the stability typically associated with aromatic compounds. While aromatic compounds are known for their enhanced stability and resonance energy, anti-aromatic compounds are destabilized due to their electronic structure. III : Once again, the distribution of the pi system is relatively stable. So this is also reactive. IV : THe distribution of the delocalized electrons makes this more stable than II, and thus more reactive.