Step 1: Understanding Kant’s Moral Philosophy.
Immanuel Kant, a central figure in modern philosophy, is known for his deontological ethics, which emphasizes duty as the basis of morality. His maxim "Duty for Duty's Sake" means that actions are morally right if done out of a sense of duty, regardless of the consequences. This is foundational to his categorical imperative, which asserts that one must act according to principles that can be universally applied.
Step 2: Analyzing the Options.
- 1. Aristotle: Aristotle’s ethics focuses on virtue and the development of good character, not on duty for duty’s sake.
- 2. Plato: Plato’s philosophy is centered on justice, the theory of forms, and the ideal state, but not specifically on the notion of duty for duty’s sake.
- 3. Socrates: Socrates focused on virtue and the examined life, but did not frame his ethics around duty for duty’s sake.
- 4. Kant: This is correct. Kant propounded the idea that morality is based on performing one's duty, irrespective of the consequences.
Step 3: Conclusion.
The correct answer is 4. Kant.
Final Answer:
\[
\boxed{\text{The correct answer is 4. Kant.}}
\]