According to the Tamil Sangam texts, the large landowners were the Vellalars. The Vellalars were an upper-caste agricultural community in ancient Tamil society. They were considered to be the highest-ranking among the four dominant castes, which also included the Brahmins (priests), Kshatriyas (warriors), and Vaishyas (merchants).

The Gahapatis, mentioned in option 1, were not the large landowners in Tamil Sangam society. They were a class of householders who were engaged in agricultural activities but did not possess large amounts of land.

The Uzhavars, mentioned in option 2, were cultivators or farmers who worked on the land but were not the owners of large estates.

The Adimais, mentioned in option 3, were slaves or servants and did not own any land.

Therefore, the correct answer is option 4, the Vellalars, who were the large landowners in ancient Tamil Sangam society.