Correct orderal acidity — IV > I > II > III
In (IV), due to presence of three highly electron withdrawing -NO2 group, (IV) has highest acidic character among them.
In (II) due to presence of one -CH3 (electron releasing) group. It has low acidic character them (I).
In (III), due to presence of two -CH3 group adjacent to -NO2 group, -NO2 group goes out of plane of benzene ring. That's why, it not participate in the conjugation (Resonance) with benzene ring. So, there is no -R effect of -NO2 group. There is only, -I effect of -NO2. That's why, it has lowest acidic character among them.
Hence, the correct order of acidity will be —
IV > I > II > III
