The 'Kudirai Chettis' were associated with the trading of horses in fifteenth century India for several reasons:

Specialization in Horse Trade: The 'Kudirai Chettis' were a specific community or group of traders who specialized in the trade of horses. Horses were highly prized in India during this period for various purposes, including military, transportation, and agricultural activities.

Importance of Horses: Horses played a crucial role in the military campaigns and warfare of many Indian rulers. They were used in cavalry units and were considered valuable assets. As a result, the trade of horses was a significant and profitable business.

Expertise in Horse Breeding: The 'Kudirai Chettis' were known for their expertise in breeding and trading horses. They were involved in sourcing horses from various regions, including foreign lands, and selling them to Indian rulers and nobility.

Lucrative Trade: The trade of horses was highly lucrative, and those involved in this business held an important position in the economic landscape of fifteenth century India. Their specialization in this trade allowed them to accumulate wealth and influence.

So, the correct option is [4].