You should be using the free Sony software designed to work with and transfer these files. Sure, you can spend $90 on a codec to use the MP4 files directly, but that raises a concern with me. That concern: if you use the MP4 files natively, you’re likely to dig them out of the BPAV folder and make one heck of a mess of things.
The BPAV folder structure may look like a bunch of gobbledygook in the Finder, but it has a definite structure. The software designed to work with these cameras uses that structure to make sense of the footage. Breaking that by poking around in there or moving/removing MP4 files is asking for trouble.
If you need to use those files on something other than a FCP system down the road, and need to export MXF files, you will run into issues unless the BPAV folder remains intact. Yes, there are ways of rescuing MP4 files and building a new BPAV folder, but why not just do the research and use the proper workflow to start with?
Step 1: review shots with Clip Browser, copy/move as needed
Step 2: transfer shots to FCP in Quicktime format with XDCAM Transfer
Step 3: archive complete intact BPAV folders for those “that’ll never happen” scenarios that happen later
Cf