The fourth principal energy level in the standard atomic model differs from the three preceding levels in that it is the first level onward to have an f-sublevel and orbitals. The number of electrons that can occupy the fourth level obeys the two-n-squared occupancy rule, 32 electrons.
Principal energy levels are defined by the principle quantum number, which may be assigned an alphabetical value. The principal quantum number can have only positive integer values and is directly proportional to orbital size and the probability of an electron being further from the nucleus. A larger principal quantum number corresponds to a larger electron potential energy, making it easier for this electron to leave the atom.
The n in the two-n-squared occupancy rule is this principal quantum number. As the quantum number increases, the energy and number of the electrons in the corresponding level also increases. Because each electron in a further and more populated energy level has more potential energy than electrons closer to the nucleus, a filled higher energy level always has more energy than a filled inner level.
Electrons are more likely to be found in some regions of an energy level than in others. These regions of high likelihood of occupation have specific shapes and are called orbitals, which are grouped together in sublevels. The first energy level has only an s-orbital. The second level has an s and a p orbital. The third level has s, p and d orbitals. The fourth level is the first to feature s, p, d and f orbitals.