What is a 4K player? What would you be connecting it to? What are you ‘playing’?

In general, if your Sonos device is designed to connect to a TV, that means it’s designed to use the data generated by the TV to play, no ‘player’ would create an ARC signal. 

I assume you purchased a 4K Ultra HD disc player. I own a Panasonic. Connect your player to any available HDMI input. Assuming you have a Sonos soundbar with HDMI connect it to a HDMI eARC or HDMI ARC port. The latter will not pass Dolby Atmos audio. Using your TV remote select the input for your player to view a disc.

ARC stands for Audio Return Channel, meaning it is not a direct input, it is “returned” to the soundbar/receiver by the TV or extractor.  Therefore, no player/streamer can directly produce HDMI-ARC audio, it must come from either an extractor that specifically extracts HDMI-ARC, or the TV’s HDMI-ARC port.

Long story short, connect your Sonos to the HDMI-ARC/eARc port on the TV and connect any players/streamers to one of the other HDMI ports on the TV.

Thanks for the replies everyone. Yes I have the sonos arc, sub and 2x 1sl's which im going to change for era 300s soon. I have got the panasonic ub820 player so just connect that to any of my hdmi outputs? Thanks very much for the helpful advice all

Yes. The Sonos remains connected to your TV (where the ARC is generated) ARC port. Any other device connects to the other HDMI ports on the TV.