I don't know too much about Pixhawk 4 but I just received a Pixhack5. AFAIK the Differences are more likely to find in the mechanical construction. Pixhack 5 has a damped IMU Unit inside the silver cube, while Pixhawk 4 has it soldered on the main logic board.
Pixhack 5 also has a built in RGB Status LED around the Cube and more I2C Connections. I think it is also heavier (90gr) as for the Pixhawk I find very different weight Specs (ranging vom 15gr to 98gr - however the lower 15gr is almost impossible, as a naked mRo X-2.1 has about the same weight with half the size and it does not have any sort of cover).
Pixhack 5 comes with a full cable set, as well as a WiFi 2 Dongle, PowerModule, Usb 2 Serial Board and I2C/CAN Distribution Board, GNSS is optional. I paid approx. 270.- U$ w.o. GNSS.