full manual is here
https://docs.sony.com/release/KDL55W800 ... _EN_FR.pdf
HDMI IN 2/3/4
If connecting a digital audio system that is compatible with Audio Return Channel (ARC) technology, use HDMI IN 4. If not, an additional connection with DIGITAL AUDIO OUT (OPTICAL) is necessary.
HDMI IN 1/MHL
You can connect your MHL (Mobile High-definition Link) device using an HD quality MHL cable.
What is MHL?
What is ARC?
Any help or guidance.