I don’t think Handbrake handles HDR at all, so it doesn’t do any tonemapping from Rec/BT 2020 to Rec709, hence the washed out colours on the 4K rip. To handle 4K rips properly, you’ll need to make sure you’re encoding to a 10-bit colour or higher format (if you have the hard drive space, consider DNxHR HQX or 444), to preserve all the colour information. Then you’ll have to tonemap the movie yourself in your editing software. I think Davinci Resolve has various built in LUTs for this, and there are a few that were made for the new OT Blurays at The Star Wars Trilogy forums. Alternatively, you could try ffmpeg using some of the settings discussed in this thread, which would allow you to tonemap the movie down to SDR and convert it in a single step. As an alternative, there are also some already-tonemapped “10bit SDR” encodes of 4K Blurays floating around on the internet - you’ll have some generational quality loss if you use these as a source, but they’re very convenient.