1921 Persian coup d'état, known in Iran as 3 Esfand coup d'état , refers to several major events in Persia  in 1921, which eventually led to the establishment of the Pahlavi dynasty as the ruling house of the country in 1925. The events began with a coup by the Persian Cossack Brigade headed by Reza Khan, and directed by the British, on 21 February 1921. With this coup Zia'eddin Tabatabaee took over power and became Prime Minister. The coup was largely bloodless and faced little resistance. With his expanded forces and the Cossack Brigade, Reza Khan launched successful military actions to eliminate separatist and dissident movements in Tabriz, Mashhad and the Janglis in Gilan. The campaign against Simko and the Kurds was less successful and lasted well into 1922, though eventually concluding with Persian success.
Question: How many years did it take for the Pahlavi dynasty to be in power?
Answer: 4