[Tom Jackson]

Are all of your mp4 files with the same codec settings?

Try the following page, these might get you going in the right direction:

https://www.appgeeker.com/how-to/convert-mts-to-mov-mac.html

You can try to convert your canon mts files to a QuickTime ie .mov format, which developed by Apple, while Final Cut Pro 7 is also designed from Apple Inc.

P.S, you may also take a try with re-encode your unreadable mp4 files with h.264 codec to check if works in your FCP 7.