What distinguishes Class 4 from Class 3 in Black's classification?

Class 4 cavities in Black's classification are characterized by carious lesions that involve the incisal edge of anterior teeth, which is a key difference from Class 3. Class 3 cavities only involve the proximal surfaces of anterior teeth and do not extend to the incisal edge, making the two classifications distinct based on this involvement.

By recognizing that Class 4 includes this edge involvement, it highlights the increased complexity and visibility of the restoration needed, especially in anterior teeth where aesthetic considerations are important. Thus, the correct answer accurately focuses on this unique feature that differentiates Class 4 from Class 3 cavities.