The Beja Congress was formed in 1952 with the aim of pursuing regional autonomy against the government in Khartoum. Frustrated by the lack of progress, the Beja Congress joined the insurgent National Democratic Alliance in the 1990s. The Beja Congress effectively controlled a part of eastern Sudan centered on Garoura and Hamshkoraib. The Beja Congress sabotaged the oil pipeline to Port Sudan several times during 1999 and 2000. In 2003, they rejected the peace deal arranged between the Sudanese government and the Sudan People's Liberation Army, and allied with the rebel movement of the Darfur region, the Sudan Liberation Movement/Army, in January 2004. A peace agreement was signed with the government of Sudan in October 2006. In the general elections in April 2010, the Beja Congress did not win a single seat in the National Assembly in Khartoum. In anger over alleged election fraud and the slow implementation of the peace agreement, the Beja Congress in October 2011 withdrew from the agreement, and later announced an alliance with the Sudan Liberation Movement/Army.
Question: About how many decades after the Beja Congress formed did it join the National Democratic Alliance?
Answer: 4