The owner of a Woburn tree service investigated after an employee was injured by a falling tree has pleaded guilty to workers’ compensation fraud. Edward App pleaded guilty in Middlesex Superior Court Monday to four counts of workers’ compensation fraud and three counts of larceny over $250. Attorney General Martha Coakley’s office alleged that App inaccurately reported his total payroll and number of employees in order to avoid paying his proper insurance premiums. The inaccuracies got App the minimum premium rate on his policy, shifting risk for undisclosed employees or injuries to the AIM Mutual Insurance Co. risk pool for workers’ compensation.