The Liberian economy had relied heavily on the mining of iron ore prior to the civil war. Liberia was a major exporter of iron ore on the world market. In the 1970s and 1980s, iron mining accounted for more than half of Liberia's export earnings. Since the coup d'état of 1980, the country's economic growth rate has slowed down because of a decline in the demand for iron ore on the world market and political upheavals in Liberia. Following a peak in growth in 1979, the Liberian economy began a steady decline due to economic mismanagement following the 1980 coup. This decline was accelerated by the outbreak of civil war in 1989; GDP was reduced by an estimated 90% between 1989 and 1995, one of the fastest declines in history. The United Nations imposed sanctions on Liberia in May 2001 for its support to the rebels of the Revolutionary United Front  in neighboring Sierra Leone. These sanctions have been lifted following elections in 2005. Upon the end of the war in 2003, GDP growth began to accelerate, reaching 9.4% in 2007. The global financial crisis slowed GDP growth to 4.6% in 2009, though a strengthening agricultural sector led by rubber and timber exports increased growth to 5.1% in 2010 and an expected 7.3% in 2011, making the economy one of the 20 fastest growing in the world. In March 2010, Bob Johnson, founder of BET, funded the first hotel constructed in Liberia in 20 years. The 13-acre  luxury resort was built in the Paynesville section of Monrovia. Liberia's external debt was estimated in 2006 at approximately $4.5 billion, 800% of GDP. As a result of bilateral, multilateral and commercial debt relief from 2007 to 2010, the country's external debt fell to $222.9 million by 2011.
Question: How many years did it take for the country's external debt to fall to $222.9 million?
Answer: 4