First I convert the video to FullHD:

$ ffmpeg -i input_video_4k.mp4 -vf scale=1920:1080 -c:a copy output_video_1080.mp4

Then I crop it:

$ ffmpeg -i output_video_1080.mp4 -vf "crop=w=607:h=1080:x=896:y=0" -c:v libx265 -crf 26 output_video_1080_insta.mp4

Sources:

https://www.reddit.com/r/ffmpeg/comments/tgkd2o/making_portrait_video_from_a_landscape_video/

https://video.stackexchange.com/questions/14907/how-to-downsample-4k-to-1080p-using-ffmpeg-while-maintaining-the-quality

• BrainDamage