SALEM – An auto mechanic about to go on trial for striking a 62-year-old Lynn woman over the head with a brick as he stole her pocketbook containing $19, pleaded guilty and received four years in state prison.Donald Blackington, 43, who last lived at 57 Fern St., in Lawrence, changed his plea a day after a 14-person jury was sworn in to begin hearing his trial.To avoid a trial, he pleaded guilty to unarmed robbery and assault and battery Thursday in Salem Superior Court before Judge David A. Lowy.The judge agreed to adopt the sentence agreement offered to the court by Assistant District Attorney Jean M. Curran and defense lawyer Michael Phelan.It was around noon on Nov. 26, 2006, when the then 62-year-old woman was walking in a parking lot near Blake Street after buying items at a local store on Union Street and she was hit from behind on the right side of her head with a brick.As she fell to the ground, the robber reached down and grabbed her purse, containing $18 in bills and another dollar in coins as the victim yelled for help.A witness saw the incident, came to the woman’s aid and called 911 as the thief fled from the scene.The witness, who was an employee of My Brothers Table on Willow Street, provided police with a description. He later called police and told them that he saw Blackington hanging around the homeless kitchen and police arrested Blackington the following day on Union Street.Lynn police officers Ralph Sirois and Stephen Withrow spearheaded the arrest and investigation.The judge credited Blackington the 479 days he has been held in custody awaiting trial.