While Ava had its hands full with the Manipuri raids in Upper Burma, a rebellion broke out at Pegu in November 1740. Ethnic Mon officials selected Smim Htaw Buddhaketi, a cousin of the king at Ava, as their king. The court at Pegu consolidated its hold in Lower Burma. Starting in 1742, Pegu, with renegade Dutch and Portuguese musketeers, began its annual raids of the upcountry. Because of the Manipuri threat, Ava could only send small armies to the south in 1743 and 1744, neither of which made any mark. The war "carried on languidly", with neither side achieving any lasting advantage until 1747. In December 1747, Binnya Dala came to power in Pegu, and the new king was determined to finish the war. He was not satisfied to gain independence for Lower Burma itself but determined to make Upper Burma its tributary. He stopped the annual raids and began planning for a decisive invasion. He sought and received French East India Company's support in firearms. Alarmed, Ava too sought support from China but no help materialised. In November 1751, Pegu launched a full-scale invasion by land and by river with a total strength of 30,000. Ava had prepared an extensive defensive line around Ava—a riverside fort at Sinbyukyun on the Irrawaddy, and a series of forts at Sintgaing, Tada-U and Pinya en route to Ava. By mid-January, the invasion forces had overcome Ava's defences, and laid siege to the city. On 21 March 1752, the invaders broke through the city's outer walls. Two days later, they breached the inner walls and took the city. The 266-year-old Toungoo Dynasty had fallen.
Question: How many forts protected Ava?
Answer: 4