Video:
Your receiver wont support 4K video (only supports HDMI version 1.3a) so you'll need to connect the video from the bluray direct to the tv to view at full 4K.
If you do instead try to connect the video via bluray -> receiver -> TV, I am not sure if it will work, but best case it will only show at the old lower resolution standard of 2K, worst case not see anything or the picture colours are wrong.
Audio:
You can still use connect the bluray audio direct to the receiver and as Steven says the 7.1 / 7.2 will be automatically down-scaled to what you are used to hearing.
So to get the best effect as I describe above, that means you'll need to fiddle around with inputs:
- selecting the audio source on receiver as the bluray player
- and selecting the video source on tv being the bluray player.
That might get annoying over time until you upgrade the receiver to handle 4K video (minimum version HDMI 1.4 I believe) and cable both audio and video cables through it