The region of Awadh (also known as Oudh) was known as the 'nursery of the Bengal Army' from the 1840s onwards because it served as a significant recruiting ground for soldiers in the Bengal Army during the British colonial period. Awadh, located in the northern part of India, provided a steady supply of recruits, particularly during the Company rule and later the British Raj. The Awadhi soldiers were highly regarded for their martial skills and loyalty, which made them sought-after recruits for the British military. This region played a vital role in maintaining the strength of the Bengal Army.

So, the correct option is [4].