It's really difficult to give substantial advice because we don't know the following:
- the quality of original source material
- the type of action (fast like sports, slow like an 80-year-old's birthday celebration)
- the final application of the finished video file (upload to YouTube, copy to an iDevice, burned to optical media)
Since we know none of this I can only give the most generic advice.
Let iMovie render in 4K Best Quality then transcode with Handbrake to 1080p.
I would use H.265 10-bit (x265) at a constant 8000 kbps video bitrate, 2-pass (no turbo first pass). Assuming it is generic 2-channel stereo audio, this should result in a file about 2.8 GB in size. Use passthrough audio if at all possible.
Then it is up to the OP to decide if the quality is "good enough" [sic] for whatever nebulous purposes he/she has.
For some 1080p transcodes, I use a 4000 kbps video bitrate, but this is for very specific low-motion content like a symphony orchestra's concert performance. For these sort of transcodes, I will definitely favor the best audio quality (like DTS-HD MA passthrough).
Again, without knowing what the original material is, it is impossible to make a specific recommendation.