As Tsushima had suffered greatly from its loss of trade with Korea as a result of the invasions, Yoshitoshi of the Sō family, then dominant in Tsushima, undertook the lead in the peace negotiations by Japan. He sent four peace missions to Joseon in 1599 to normalize relations. The first three were captured and sent directly to Beijing by Chinese troops, but the fourth one, in 1601, successfully obtained from the Joseon court the promise of a normalizing of relations upon the return of remaining Joseon captives. As Ming troops continued to be present in Korea following the withdrawal of Japanese forces, the major incentive for Joseon for the normalization of relations with Japan was the withdrawal of the Chinese soldiers from their territory. The Ming Chinese themselves were causing as much havoc as the Japanese had during the actual conflict, and their presence continued to strain Joseon's national economy and infrastructure. In response to the Joseon request, Yoshitoshi promptly released several Joseon prisoners and between 1603 and 1604 helped the Joseon envoys to repatriate a further 3,000 by organizing negotiations at Kyoto with Tokugawa Ieyasu, by then the Shogun of Japan. In the continuation of the diplomatic talks toward peaceful relations, Joseon in 1606 expanded its conditions and demanded that the Shogun write a formal letter requesting peace, and to extradite the Japanese soldiers who had defiled the Joseon Royal Tombs near Hanseong . Realizing that the Shogunate would never agree to such a request, Yoshitoshi sent a forged letter and a group of criminals instead; the great need to expel the Ming soldiers pushed Joseon into accepting and to send an emissary in 1608. The end result was a return of Joseon prisoners and the restoration of diplomatic and trade relations between the two countries.
Question: How many years passed between the sending of the four peace missions to Joseon and the obtaining of the the promise of normalizing of relations?
Answer: 2