You’re not getting 4K HDR ( true, full HDR ) recording on a iPhone. You my get “rich tone”, somewhat expanded dynamic range, but the iPhone is not able to record rec2020 color space and 10 bit, both of which are required for true “ HDR” recording.
There is, currently, only one phone on the market that’s able to record true 4K hdr.
[doublepost=1525519736][/doublepost]If you want fantastic 4K video then use FiLMic Pro, record in log gamma and color grade the video in post using something like LumaFusion.