The 975 won't pass through 4k. I tried a new FireTV 4k box connected to the 975 and out to my Vizio into the HDMI2.2 input and
the picture displayed only in the upper left quadrant of the screen.
Thankfully this Vizio has one HDMI input that utilizes an internal upscaling engine(which is why I chose this one). I switched to that input and now I get an upscaled picture that fills the screen. Works very well.
In lieu of that, you would need to connect directly to the TV and return the audio back to the 975 either through an HDMI cable connected to the HDMI-ARC of the TV, or from an optical output, if your TV has one.
Some 4k Blu-Ray players have separate HDMI jacks, one main HDMI and the other for audio only. But, that is only a single source.