I'm running Mojave with iMovie 10.1.12. So an older version than yours.
Could you post a screen shot that shows what you are seeing? I have never noticed 4k displaying at only 70-75% of the frame unless the Ken Burns view was activated. How did you determine that?
iMovie has a preset 16:9 screen dimension that cannot be changed. It will will fully display a 4:3 aspect ratio clip with the Crop to Fit setting, but there will be black bars on each side to make it fit the 16:9 dimensions of the iMovie screen. Clips with odd aspect ratios may be cropped to fit the screen. Clips that exceed the 16:9 aspect ratio will be cropped to fit the 16:9 screen, or there may be letter boxing so that it displays fully.
TVs have their own aspect ratio settings, and may display the same clip differently to fit the screen. On your computer screen you should be able to do View/Enter Full Screen and have the clip display full screen. Likewise in iMovie if you click on the full screen symbol in the lower right corner of the preview screen it will play full screen. But, again, a 4:3 clip would have black bars on each side to make it fit the computer screen.
-- Rich