I would prefer to have mp3 files, with the chapters numbered 01, 02, 03 etc to avoid having chapter 11 play immediately after chapter 1 etc. Simple is best. I don't use fancy apple or other audio players, but am happy pushing my mp3 files from my phone to my vehicle stereo via an aux cable, or burning them onto a single cd rom and putting that into the cd player.
I am very familiar with using mp3 files, but if it came as some fancy audio book file (is that what the "m something 4" file is?) then I'd have to learn something new, and I'd prefer to not have to do that. Okay, I'm a bit older, and admit I can get bit grumpy about complex technology stuff. That's my opinion.