When it "converts" it is not ruining the video, but it is tonemaping the HDR metadata to SDR video. madVR does similar but in highest quality possible if you set it up so. Just follow the offical threads here on doom9 and also on AVS.
What does not work for you in 4K? I have several 4K screens and every one works just fine with pure MPC-BE without external Filters like LAV Filters and madVR and also with the external filters. I need info to help you with that.
The Envy does also the tonemapping. Look up its price.
By the way I have an OPPO too, but it is a UDP-203