In the following compounds

the order of acidity is

In case of substituted phenols, Presence of electron with drawing group(-NO₂) increases the acidic nature due to I-effect and Mesomeric effect. where as electron donating group(-CH₃) decreases acidic nature.

p- Nitrophenol is a stronger acid than o-nitrophenol.
Because intramolecular H-bond makes o-isomer weaker than p-isomer.
In the o-isomer, due to intramolecular hydrogen bond, the H atom of OH group is tightly attached and is difficult to remove. This decreases the acidity of o-isomer. 

Hence the order of acidic nature is, IV > III > I > II