iMovie only imports 4 seconds of a movie clip that is much longer than 4 seconds
If you try to drag and drop a video from photos, for whatever reasons, it only takes it in as an image, and the image is displayed for only 4 seconds by default. If you would like to import into iMovie as a video, do the following steps:

In photos, select your photo, click on 'File' at the top menu, and click on 'Export'. You can export the video or the unmodified original version of it. Confirm video resolution settings. Once you click on export, select a destination to export to.
Once the export is complete, drag and drop from finder into iMovie.