The correct sequence of cities in South-East Asia as one proceeds from south to north is Jakarta (3), Singapore (4), Bangkok (1), and then Hanoi (2). This ordering reflects their geographical positions: Jakarta is located on the island of Java, Indonesia, to the south; Singapore is just north of the equator; Bangkok is in Thailand further north; and Hanoi is in northern Vietnam. Understanding the geographical layout of Southeast Asia is crucial for the UPSC Prelims as it underscores the region's cultural, economic, and political significance, as well as the interplay of geography with historical trade routes and colonial influences in the context of Indian and global history.