The dehydration of alcohols under acidic conditions proceeds via an E1 mechanism involving the formation of a carbocation intermediate. The rate-determining step is the formation of this carbocation. Therefore, the alcohol that forms the most stable carbocation will be the most readily dehydrated.

Reactivity Order:

1. Carbocation Stability: The stability order is Tertiary ($3^{\circ}$) > Secondary ($2^{\circ}$) > Primary ($1^{\circ}$).
2. Resonance: Alcohols that form allylic or benzylic carbocations are even more reactive due to resonance stabilization.
3. Electron Withdrawing Groups: Presence of electron-withdrawing groups (like $-NO_2$ or $-C=O$) adjacent to the carbocation center destabilizes it, reducing reactivity.

Option 4 likely contains a structure that yields a stable tertiary carbocation or one stabilized by resonance, making it the most reactive.