RPG400, or RPG IV, is a high-level programming language used primarily for business applications on IBM's iSeries (formerly AS/400) platform. It is an evolution of the earlier RPG languages and includes features that support modern programming practices, such as free-form syntax and improved data handling. RPG400 is commonly used for developing and maintaining enterprise resource planning systems and other business-critical applications. Its integration with database systems allows for efficient data processing and reporting.