Step 1: Understand Koontz and O'Donnel's view.
Koontz and O'Donnel proposed four main functions of management, which include planning, organizing, leading, and controlling.
Step 2: Evaluate the options.
- They argued that these four functions form the core responsibilities of managers in any organization.
Step 3: Conclude.
Thus, the correct answer is 4.
Final Answer:
\[
\boxed{4}
\]