4K HDR Encoding Setting

[Selur]

will be great if someone is writing down an step by step guide

No step-by-step but here are some pointers. May be someone else will make a video or write some step-by-step instructions.

Can everyone tell to me what is the correct 4K HDR setting in this program?

Correct HDR settings depend on the color grading parameters used for your content.
Typically you would:

• enable x265->Signaling->General Signaling->HDR
  
• disable the settings under x265->Signaling->Adjust color
  
• set the settings under x265->Signaling->Video Usability Information to the requirements of your source
  
• Make suer your set the x265->Base->Compatibility Restrictions
  

How can I re-encode 4K HDR videos with x264 to 1080p?

In general you can load the source resize the source and reencode the source.
But since you probably also want to do a HDR->SDR conversion you probably might want to wait for the next release since Hybrid (then supports ffmpegs 'tonemap' filter which will be available under Filtering->ToneMap).

Because the preview script window can not showing properly the colors and the final encode will be an .... not good

That probably happens because you are doing a HDR->SDR conversion without tone mapping.

Cu Selur