Not sure about the HDR differences between resolutions, but I recall that 4K30 mode might have enhanced lighting implemented for headlights that causes other vehicles and objects to cast shadows. Unless that's exclusive to the PC version. I honestly can't remember.Me 2. They will definitely do it.
I play FH4 on the XOX in the 1080 60 mode and it looks quite good already in motion.. Increasing the resolution with much better drawdistances and shadowdetail etc could make it stunning. I think i also read that the 4k 30 graphical setting had a more advanced form of HDR versus the 1080 performance mode.
Maybe introducing some new form of Raytracing even. Advanced reflections on the paint of the cars etc...
In either case, yeah, as I was playing the game yesterday, the draw distances on LODs and shadows were definitely noticeable. It's probably just the excitement surrounding Xbox Series X getting to me, but I think FH4 is already such an incredible game, and I can't wait for it to look even better on Xbox Series X.