What is an AVR?

by AVR do you mean an Audio Receiver then all you would need to confirm that the HDMI input/outputs are HDMI 2.0 standards if you want 4k at 60 hz which i would for smoother playback... HDMI 1.4 High speed would do 4k but only at 30hz...

but if you are worried about compatibility i would just connect an optical Audio cable from the MI box to your AVR box and do it that way...

4 easy steps to 4k playback ... (1) Display or Tv needs to be a 4k-2k display { Ultra HD. UHD is a resolution of 3840 pixels × 2160}

(2) you well then need an HDMI 2.0 4k-2k @ 30-60 hz cable High speed- the included cable in the box is HDMI 2.0

(3) 4k media // on USB drive for local playback''' or Netflix for Streaming 4k''' --- UHD account required... for netflix

(4) High-speed internet connection at least 25-to-70 Mbps Download speed or higher ...

Sure by today's standards I would want my AVR to be 4k ready however you don't need that to get 4k to work just connect the Mi box directly to the TV and plug in the audio into the AVR with the optical Audio cable. You should be fine with that

Then in that case I would just use optical audio cable between the mi box and the receiver make sure the optical audio cable has the little adapter mini toslink with it for the mi box.

I found this cable ... https://www.amazon.com/dp/B00T8HWUVS/ref=cm_sw_r_cp_apa_yM-hybBYH846N