The fourth planet from the primary of the Nivenia System, Nivenia Secondus is a gas giant mainly used for procuring hydrogen to build ringworlds with. Its huge amount of surface area and high temperatures lead to a remarkably high population (in terms of general planetary colonies, second only to Nivenia Prime), mainly living in aerostat habitats deep inside the atmosphere. However, orbital rings provide support to thin bands of habitats that want to join together to form larger districts.
With such a huge number of inhabitants, the planetary coordinator of A4 enjoys the same limited autonomy granted by the Nivenian Empire that most ringworld coordinators have, but that the planetary coordinators of less populous worlds, such as A11, do not.
A4 also supplies a large portion of the organics necessary to keep the ringworlds working, as its lower atmosphere's unique conditions allow for extremely cheap chemical processing (hot, high pressure hydrogen atmosphere).
The name "Nivenia Secondus" was given to this planet over two hundred years ago due to its position as second most populous planet in the system.