MP4 files are compressed using H.264/MPEG-4 codec which is a final delivery codec not use for editing. Therefore you may have problems when importing MP4 footage into Final Cut Pro. For smoothly editing MP4 files in Final Cut Pro, you first need to convert MP4 to a more editing-friendly format, such a Final Cut Pro's ProRes.
To do the MP4 to Prores Conversion, as well as to keep your video quality, you need a professional HD Video Converter for Mac to be your helper. Here Pavtube HD Video Converter for Mac is highly recommended. The one used here can meet your demands with its simple interface but rich functions. With it, you can effortlessly transcode H.264/MPEG-4 MP4 files to ProRes 422 encoded MOV for Final Cut Pro.
Meanwhile, the Mac Video Conversion tool can create files compatible with various non-linear editing systems like Avid, FCE, iMovie, and Adobe Premiere (Pro) etc. Just download it and follow the guide and go on your movie director experience.
Guide: Transcoding H.264/MPEG-4 MP4 to ProRes 422 codec for Final Cut Pro
Step 1: Add mp4 footage from MP4 file folder to Prores converter for FCP. You can directly drag and drop video files to this software.
Step 2: Choose Final Cut Pro editing codec.
Select to rewrap your mts files to "Apple Prores 422(*.mov)" or "Apple Prores 422(HQ)(*.mov)" to work fine with FCP 7.
Step 3: Hit "Convert" button to carry out the conversion process. When the conversion ends, you can run the Final Cut Pro and click File --> Import --> Files to import the resulted MOV files and edit them in FCP easily, you can follow this import video to fcp guide.
From the steps above, this software will solve you MP4 files conversion and editing problems, and then you can import and use your H.264/MPEG-4 MP4 files in FCP on Mac without any problems.