The pKa value for the conjugate acid of 4-nitrophenol is approximately 7.15. This value indicates the acidity of the compound, with lower pKa values representing stronger acids. In the case of 4-nitrophenol, the presence of the nitro group -NO2 at the para position increases the acidity due to its electron-withdrawing nature, which stabilizes the negative charge on the phenoxide ion formed after deprotonation.