Step 1: What controls acid strength.
An acid is stronger when its anion (the leftover after losing H$^+$) is more stable. Groups that pull electrons away (like -NO$_2$) stabilise the anion and raise acidity. Groups that push electrons in (like -CH$_3$) lower it.
Step 2: Rank the substituent effects.
So the trend in acidity follows: nitro substituted acid is strongest, the plain unsubstituted acid is in the middle, and the methyl substituted acid is weaker. Phenol type compounds with no -COOH come out the weakest of the set.
Step 3: Apply this to the given compounds.
Reading the labelled compounds, IV carries the strongest electron withdrawing group, I is the reference acid, III has a mild electron donating effect, and V is the least acidic.
Step 4: Build the order.
Putting them from strongest to weakest gives IV > I > III > V.
Step 5: Conclusion.
So the answer is IV > I > III > V. \[ \boxed{\text{IV > I > III > V}} \]