I don't think mp4 and mp4a are the same thing.
I did not go through all the info but it seems the 'a' stands for only audio.
Bridge can handle a lot of file type but by far not everything. It should handle all Adobe Suite possible filetypes but even this is still not the case. However trying to get all kinds of file types handled will be a big task
Not sure what you are trying to do. You can not just change an extension to make a different one. You need to run it through a converter.
You can set VLC to be default program to open video file, as I have. Works well. But believe it still plays thumb in Quicktime.
I'm trying to get VLC to play mp4 files that i double click on in Bridge.
Bridge doesn't seem to have "mp43" in the list of file types in the "File Type Association" box
So my mp4 open with Winamp.
Why is there no "mp4" in the list of files in the "File Type Assocation" preferences box?