I am waiting to see what HDR 4k Looks like. Even if the picture resolution isn't noticeable, if color is supposed to be that much more accurate, you will be able to notice that from a distance.
I got my 4k TV for $1800 65" which is not that far off a comparable high end 1080P TV. It was an 850B, but was then upgraded by my Dealer when I had a burnt pixel to the 850C. I agree on distance and noticeable picture quality, but with the newer models which are pretty much all 4k, you will usually get higher contract ratios, faster CPUs inside if you have a Smart TV and more features.
Does that mean you should go out and buy a 4k now, definitely not, wait till HDR is more readily available.
the Oppo has Pre-Outs to the receiver, so you wouldn't necessarily have to use an optical cable if you have inputs on your receiver