Premiere only crashes when the video-files are larger than 1080p, i scaled the 4K videos down to 1080 and they worked.
...everything crashed, when the video-files were larger than 1080p, but 1080p or less worked
This is not normal behavior.
Premiere is fully capable of importing and editing 4k media.
I´m working on an 2011 i5 iMac and an external Hard Drive for my Videos.
Try moving your media files to an internal drive before importing.