Answer : 4. "हरदोई"
Explanation :

1. The Indo-Gangetic plain, where the internal flow reemerges, is locally known as the Terai region. It is a low-lying area with fertile soils to the north of the Indo-Gangetic plain.

2. The Terai region has been formed from the sediments of the rivers originating from the Himalayas. These rivers enter the plains after emerging from the Himalayas and bring with them a huge amount of sediment. These sediments accumulate in the northern part of the plain and form the Terai region.