The correct order of the town/city in ascending order of their size and functions is:

(4) (B), (A), (C), (D)

Explanation:

• (B) Town: Generally smaller than a city, with a lesser population and fewer functions.
• (A) City: Larger than a town, typically with a higher population and more functions.
• (C) Conurbation: An extended urban area formed by the growing together of several towns or cities.
• (D) Million City: A city with a population of one million or more.

Therefore, the correct order is (B), (A), (C), (D).