The sub-system consisting of 3 (three) first equations has a unique solution x = y = z = 2. When you add the fourth equation, it becomes 2a + 2b + 2c = 0, or a + b + c = 0. Thus, a) if a + b + c = 0, then the system of 4 equations has a unique solution. b) if a + b + c =/= 0, then the system of 4 equations has no solution. c) it is IMPOSSIBLE to this system to have infinitely many solutions.