Kimani Washington, the key witness in a quadruple slaying in Mattapan, is expected to plead guilty Friday to several charges connected to the killings of four, including a 2-year-old boy. The most serious charges are armed home invasion, armed robbery, and armed carjacking. Washington, 37, agreed to testify against Dwayne Moore and his cousin, Edward Washington, in exchange for a lighter recommended sentence of 16 to 18 years. Washington could have faced life in prison for his role in the robbery for cash and drugs that led to the 2010 fatal shootings on Woolson Street in Mattapan. Edward Washington, 33, was acquitted in March, but Moore, 35, was convicted last month of first-degree murder.