In the context of organic chemistry, the classification of molecules based on functional groups plays a crucial role in understanding their reactivity and properties. Consider the following compounds:

Compound 1: C₄H₁₀O (butanol)
Compound 2: C₄H₈O (butanone)
Compound 3: C₄H₄O₂ (butanoic acid)
Compound 4: C₄H₆O₂ (butenoic acid)

Each compound is classified based on its functional group. Which compound among the four exhibits a carboxylic acid functional group?