Arrange the following in the correct order of their acidic strength:
I. \( \text{C}_6 \text{H}_4 (\text{OH}) \) (I)
II. \( \text{C}_6 \text{H}_5 (\text{OH}) \) (II)
III. \( \text{C}_6 \text{H}_4 (\text{NO}_2)(\text{OH}) \) (III)
IV. \( \text{C}_6 \text{H}_4 (\text{NO}_2)(\text{OH}) \) (IV)