In Chapter 4 of Founding Brothers, what attempted plot did Washington's officers devise?
The attempted plot that you are referring to is most commonly known as the Newburgh Conspiracy after the town in New York State in which the army was quartered in 1783.
The Newburgh Conspiracy came about largely because the Continental Congress lacked the money to be able to pay many of the officers in the army. This had been a problem throughout the war. The officers, unhappy with Congress on this account, were planning to send a letter to Congress threatening serious consequences (perhaps involving a military takeover) if Congress did not give the army what it needed.
Washington quashed the “conspiracy” by stating his support for Congress and for the idea of civilian (not military) government.