The longest era lasting about 4 billion years was what era?

The Precambrian Super-Eon is the informal name for the time on earth before the Cambrian Period. It lasted from 4,576 millions years ago until 542 million years ago, about 4.034 billion years.

No geological era lasted as long. The longest era was the Paleoproterozic Era(2,500 - 1,600Ma) which lasted 900 million years.