Electrons move between principle energy levels primarily through excitation by external energy sources such as heat or radiation particles.
An “excited” state is always unstable, and the electron will return to its more stable level with the release of energy, usually in the form of light. The sub-shells are similar in type (s,p,d,f) and geometry in all principle levels (1,2,3,4). Only their energies relative to the nucleus are different.
See also: http://chemistry.tutorcircle.com/inorganic-chemistry/electronic-configuration.html
And here: http://chemistry.tutorvista.com/inorganic-chemistry/electronic-configuration.html