I doubt you will get better 4k upscaling from an av receiver compared to an oppo 4k player.
If you have a panasonic display and panasonic 4k player then you have some options to achieve the best image.
There is a you tube video from hd tv test demonstrating this
The guy who calibrated my TV recommended the Tv to always be the main upscale and set my av reciever to pass through the signal, and the BD player to have all the processing switched off.