Taxila was the capital of Gandhara among the 16 Mahajanapadas. The Mahajanapadas were ancient kingdoms or republics in India during the 6th century BCE. They were the dominant political units of the time and played a significant role in shaping the political landscape of ancient India.

Option 1: Kosala was a different Mahajanapada with its capital at Shravasti.

Option 2: Kuru was another Mahajanapada, but its capital was Hastinapur.

Option 3: Vajji was also a Mahajanapada, but it had its capital at Vaishali.

Option 4: This is the correct answer as Taxila was indeed the capital of Gandhara, which was one of the Mahajanapadas.

Therefore, the correct answer is option 4.