Organic Chemistry – Some Basic Principles Part 1

Acidity of phenol (I) is weaker than any carboxylic acid.
Electron withdrawing nature (−R,−I), of −NO2 group at para position increases acidic strength of (II), whereas + l effect of −CH3 group at para position decreases acidic strength of (IV).
So, the order of acid character is II > III > IV > I.