The four letter answer to the crossword is 'ERAS'. ERAS stands for Epochs, Reigns, Ages, and Stages. These are all terms used to describe important times in history. An epoch is a period of time marked by significant events or changes, a reign is the period of rule of a monarch, an age is a period of time marked by certain characteristics, and a stage is a period of development. All of these terms can be used to refer to important times in history.