There are 4 papers in the BPSC (Bihar Public Service Commission) mains exam. Let's discuss each paper in detail:

1. General Hindi (Qualifying Paper):
- This paper is of qualifying nature and is designed to test the candidates' understanding and command over the Hindi language.
- It consists of two sections: Section A and Section B.
- Section A includes questions related to Hindi grammar, vocabulary, comprehension, and writing skills.
- Section B is an essay writing section where candidates are required to write an essay on a given topic.

2. General Studies Paper 1:
- This paper aims to assess the candidates' knowledge and understanding of various subjects.
- It covers a wide range of topics such as history, geography, polity, economy, science, and current affairs.
- The questions are objective in nature and candidates need to choose the correct answer from the given options.
- The paper is designed to test the candidates' analytical and problem-solving skills along with their knowledge of the subjects.

3. General Studies Paper 2:
- Similar to General Studies Paper 1, this paper also covers a wide range of topics including history, geography, polity, economy, science, and current affairs.
- The questions are objective in nature and candidates need to choose the correct answer from the given options.
- The paper focuses on testing the candidates' analytical and problem-solving skills along with their knowledge of the subjects.

4. Optional Subject Paper:
- In this paper, candidates have to choose one optional subject from a list provided by the BPSC.
- The optional subjects include various disciplines such as history, geography, political science, sociology, economics, mathematics, and so on.
- Candidates need to choose the subject they have expertise in and are comfortable with.
- This paper consists of subjective questions where candidates need to write detailed answers to the given questions.

These are the 4 papers that candidates need to appear for in the BPSC mains exam. It is essential to prepare thoroughly for each paper to increase the chances of scoring well and securing a good rank in the exam.