Using the newest version of PPro on Mac, there is an error when trying to folders of files. The MP4 files themselves import fine.
We have a folder that contains sub folders of different lens flares. The files themselves are MP4. Using the import command, drag and drop import and using the media browser to import the Parent folder causes an error and only some of the contained files, if any, to be imported. Importing the files themselves (not from a parent folder) works just fine. Seems to be a bug with the 2018 update. 2017 handled folders just fine.
What macOS version are you on? If you are on 10.11 then I would recommend updating to a newer version as we have found that to resolve importing issues in many cases that are new to the latest update of Premiere. If you do update you will also need to clear your media cache before re-importing files that have previously failed.
The media cache can be found here:
Mac: /Users/%username%/Library/Application Support/Adobe/Common
More details can be found here: