Technically P4 uses AC power to run DC motors to run train. P7, P5 are pure AC locos that use AC power (for 3 phase traction motors) to deliver much more efficiency AND power or tractive efforts. Obviously, power delivered and efficiency at which it is done has forced P4 to be out of production. But don't worry, P5 and P7 may see production cuts in favour of MEMUs, VBs (T-18/19/20/21 or later).