Choose the incorrect statement.

The relative ease of dehydration of alcohols on heating with a protic acid follows the order: Tertiary $>$ Secondary $>$ Primary. This is because the dehydration mechanism involves the formation of a carbocation intermediate, and the stability of carbocations follows the order $3^{\circ} > 2^{\circ} > 1^{\circ}$. Therefore, the given statement regarding the ease of dehydration is incorrect.

The reaction of alcohols with carboxylic acids and acid anhydrides is carried out in the presence of a small amount of concentrated $H_2SO_4$. The reaction is reversible, and the removal of water drives the equilibrium forward, making the first statement contextually accepted as per standard textbook language.

In p-nitrophenol, the $-NO_2$ group is strongly electron-withdrawing ($-M$ and $-I$ effects), which stabilizes the conjugate base and increases acidity. In p-cresol, the $-CH_3$ group is electron-donating ($+I$ and hyperconjugation), which destabilizes the conjugate base. Hence, p-nitrophenol is more acidic than p-cresol.

The formula $C_4H_7OH$ has a degree of unsaturation of $1$. The possible cyclic structural isomers are cyclobutanol, 1-methylcyclopropan-1-ol, 2-methylcyclopropan-1-ol, and cyclopropylmethanol. This gives exactly four structural isomers.

Answer: Relative ease of dehydration of alcohols on heating with a protic acid is Primary > Secondary > Tertiary