Step 1: Understanding the Concept:
Chronology questions require placing historical events in the order they occurred, from the earliest to the most recent.
Step 2: Detailed Explanation:
1. Russian Revolution (3): This happened in 1917, leading to the establishment of the Soviet Union.
2. Unification of Germany (2): Following the fall of the Berlin Wall, East and West Germany were formally unified in October 1990.
3. Disintegration of USSR (1): The Soviet Union formally ceased to exist in December 1991.
4. Introduction of 'Shock Therapy' (4): This model of economic transition was intensely applied in Russia and other post-communist states in the early 1990s, specifically gaining momentum around 1992 after the USSR's collapse.
The order is: 1917 (3) $\to$ 1990 (2) $\to$ 1991 (1) $\to$ 1992 (4).
Step 3: Final Answer:
The correct chronological sequence is 3, 2, 1, 4.