The sixth century BCE is indeed a significant period in Indian history, marked by various important developments. Let's assess the statements one by one:

A. It is an era associated with early states/mahajanapadas: True. This period is associated with the emergence of various mahajanapadas (powerful kingdoms) in the Indian subcontinent.

B. It is an era associated with the development of coinage: True. The sixth century BCE saw the introduction of coins in India, which facilitated trade and commerce.

C. It witnessed the growth of temple architecture and idol worship: Not necessarily. While temple architecture and idol worship did become more prominent in later centuries, the emphasis on this during the sixth century BCE is not well established.

D. It is regarded as the golden period of Indian painting: This statement is not accurate. The concept of a "golden period of Indian painting" is usually associated with different time periods, such as the Gupta period, and not specifically the sixth century BCE.

E. It is an era associated with the growing use of iron: True. The usage of iron became more prevalent during this period, leading to significant changes in various aspects of society, including warfare and agriculture.

 The correct answer is [4].