C. F. Keil and Franz Delitzsch on Proverbs 26:4

Pro 26:4 4 Answer not the fool according to his folly,    Lest thou thyself also become like unto him.

After, or according to his folly, is here equivalent to recognising the foolish supposition and the foolish object of his question, and thereupon considering it, as if, e.g., he asked why the ignorant man was happier than the man who had much knowledge, or how one may acquire the art of making gold; for “a fool can ask more than ten wise men can answer.” He who recognises such questions as justifiable, and thus sanctions them, places himself on an equality with the fool, and easily himself becomes one. The proverb that follows affirms apparently the direct contrary: