The correct order of decreasing reactivity towards an electrophilic reagent for the following compounds is: (i) Benzene, (ii) Toluene, (iii) Chlorobenzene, (iv) Phenol.

The reactivity of a substituted benzene ring towards electrophilic substitution depends on the nature of the substituent present:

1. Phenol (iv) has an $\text{-OH}$ group, which is a strongly activating group due to its strong $\text{+R}$ (resonance) effect. It increases electron density on the ring, making it highly reactive towards electrophiles .
2. Toluene (ii) has a methyl group ($\text{-CH}_3$), which is an activating group due to its electron-donating inductive effect ($\text{+I}$) and hyperconjugation, making it more reactive than benzene .
3. Benzene (i) serves as the reference compound without any substituents.
4. Chlorobenzene (iii) has a chlorine atom. Halogens are moderately deactivating because their strong electron-withdrawing inductive effect ($\text{-I}$) outweighs their resonance effect, decreasing the overall electron density on the ring. Thus, it is less reactive than benzene .

Therefore, the correct decreasing order of reactivity is Phenol > Toluene > Benzene > Chlorobenzene, which corresponds to (iv) > (ii) > (i) > (iii).