For a start, does any lore specify a fifth era? The defeat of Mehrunes Dagon was the beginning of the 4th era (coincidently the end of TES 4) and Alduin being defeated is the beginning of the fifth? I haven't finished the main quest so please forgive me.
Not anything that shows up an any games yet, but there was this:
http://www.imperial-library.info/content/loveletter-fifth-era-true-purpose-tamriel