Kunti had 4 sons - Yudhishthira (from Yama/Dharma), Bhima (from Vayu), Arjuna (from Indra), and the twins Nakula and Sahadeva (from the Ashwini Kumaras). The 5th Pandava, Karana, was also biologically her son from Surya but was raised by a charioteer. So counting only the sons she raised, the answer is 4.