Theory:

IV
     Getting off the train at Ranchi next morning, he realised at once that he had never been there before.
 
     He came out of the station, took a taxi and drove around the town for a while. He realised that the streets, the buildings, the hotels, the bazaars, the Morabadi Hill — with none of these had he the slightest acquaintance. Would a trip to the Hudroo Falls help? He didn't believe so, but, at the same time, he didn't wish to leave with the feeling that he hadn’t tried enough. So he arranged for a car and left for Hudroo in the afternoon.
Explanation:
 
Bepin began his journey toward Ranchi and arrived there the next day. As soon as he got off the train at the railway station, he realised that he had never been there before.

Bepin got into a cab as soon as he exited the Ranchi station. He drove around town for a while. The streets, houses, hotels, bazaars, and Morabadi Hill were all unfamiliar to him. None of the locations he'd seen in Ranchi had helped him recall his initial visit to the city.
 
Bepin in Ranchi.png
Bepin in Ranchi

Suddenly, an idea struck his mind. He felt a trip to Hudroo Falls would be beneficial. Though he didn't believe in this plan completely, he wanted to give it a try. After failing in all of his attempts to restore his memories, Bepin had no choice but to turn to Hudroo. He didn't want to leave Ranchi with the impression that he hadn't tried hard enough.

After all, Bepin's current situation urged him to do so. So when all of Bepin's attempts to recall the Ranchi's initial journey failed, he decided to give it one final go. Hence, he rented a car and drove to Hudroo in the afternoon, hoping a trip to Hudroo would help.
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.