What does your final output look like?
Remember that even a rendered timeline is only a preview. In fact, in the program's preferences you can set the quality of the preview to a higher level if you'd like.
But what matters is what your final video looks like. If you're using 4K 30p as your source, your project settings and your output settings, your finished piece should look as good as the original.
The only qualifier is that you're using HEVC video from an iPhone, which version 2020 may struggle with.
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