I have connected PS4 to a 4K TV via HDMI/STB port, then, TV is connected to a 5.1 Home Theatre system via HDMI/ARC port.
If I plug in a USB drive to TV and I watch a movie with 6 channel (5.1) sounds, Home Theatre System receives 5.1 audio fine via HDMI/ARC.
But it doesn't get 5.1 audio from PS4...