user10560: Looking for a ffmpeg command that would convert IphoneX HEVC 4K 25fps videos to any format that will work on PC - PremierePro and keep it lossless.

user18314: Conversion always will involve some loss, only copy (-c) is lossless. 
There are some suggestions for "lossless" encoding in this Q&A, but I can't test them on my phone. There are UpVoted and have an accepted answer: the intermediate and output file will be uncompressed, and huge.
You can minimize loss when converting your existing videos by choosing an excessively high bitrate, since it's a cellphone video it will be of a limited length and thus only expand so much. 
The ffmpeg H.264 Video Encoding Guide claims to offer lossless encoding using the -crf 0 command. It also mentions that you should expect incompatibility problems with some programs.
The section: Choose a CRF value says:

The range of the CRF scale is 0–51, where 0 is lossless, 23 is the default, and 51 is worst quality possible. A lower value generally leads to higher quality, and a subjectively sane range is 17–28. Consider 17 or 18 to be visually lossless or nearly so; it should look the same or nearly the same as the input but it isn't technically lossless. 

Using '0' isn't so much a "conversion" as it is a decompression and wrapping up the uncompressed video, the so-called converted video isn't recompressed.
You can correct the problem at it's source (in future) by:
On your iPhone, go to Settings > Camera > Formats > change "High Efficiency" to "Most Compatible".

10560: <p>Looking for a ffmpeg command that would convert IphoneX HEVC 4K 25fps videos to any format that will work on PC - PremierePro and keep it lossless.</p>
 18314: <p>Conversion always will involve <em>some</em> loss, only copy (-c) is lossless. </p>

<p>There are some suggestions for "lossless" encoding in this <a href="https://stackoverflow.com/questions/6701805/h264-lossless-coding">Q&amp;A</a>, but I can't test them on my phone. There are UpVoted and have an accepted answer: the intermediate and output file will be uncompressed, and huge.</p>

<p>You can minimize loss when converting your <strong>existing</strong> videos by choosing an excessively high bitrate, since it's a cellphone video it will be of a limited length and thus only expand so much. </p>

<p>The <a href="http://trac.ffmpeg.org/wiki/Encode/H.264#LosslessH.264" rel="nofollow noreferrer">ffmpeg H.264 Video Encoding Guide</a> claims to offer lossless encoding using the <code>-crf 0</code> command. It also mentions that you should expect incompatibility problems with some programs.</p>

<p>The section: <strong>Choose a CRF value</strong> says:</p>

<blockquote>
  <p>The range of the CRF scale is 0–51, where 0 is lossless, 23 is the default, and 51 is worst quality possible. A lower value generally leads to higher quality, and a subjectively sane range is 17–28. Consider 17 or 18 to be visually lossless or nearly so; it should look the same or nearly the same as the input but it isn't technically lossless. </p>
</blockquote>

<p>Using '0' isn't so much a "conversion" as it is a decompression and wrapping up the uncompressed video, the so-called converted video isn't recompressed.</p>

<p>You can correct the <em>problem</em> at it's source (in future) by:</p>

<p>On your iPhone, go to Settings > Camera > Formats > change "High Efficiency" to "Most Compatible".</p>
