There’s an issue with the Clip Browser that should, but doesn’t create proper BPAV folders from orphaned MP4s. It’ll be fixed in Clip Browser 1.5 (or 2.0).
On Mac
https://www.ffmpegX.com/
Will allow you to recompress the MP4 to another format.
What I then do is use that encode in Episode Pro (EX MP4 does not work in Episode Pro) and encode to XDCAM EX.
Yes, that’s a couple of rounds of lossy compression but it’ll get you an EX file.
You should charge the client for both the time to do the above as well as to research the solution. I have very little sympathy for clients who use gear and don’t read the manuals on Clip Browser or XDCAM Transfer or don’t hirer a properly trained data wrangler. Heck a Google search would have probably given them the proper work flow.
First step is I’d go to the client and ask for BPAV folders. If they don’t have them then I’d let them know that they’ll be charged to data recovery just as if they had handed you damaged tapes that you had to repair.