Can ffmpeg input mp4 and convert it to m4b or m4a?
This might be a bit late but if it helps I've got a working PowerShell script that I use to create chapterized m4b files which I am assuming you are trying to do since you list m4b. 

I use the following command for mp3 format files 

ffmpeg -f concat -safe 0 -i list.txt -vn -y -b:a 64k -acodec aac -ac 2 temp.m4a


If its already in AAC format (MP4) it's even simpler

ffmpeg -f concat -safe 0 -i list.txt -c copy temp.m4a


list.txt is a simple list of all the files to combine into a single m4a, 1 file per line. I found this to work much better than trying to do it using INPUT and listing all the files on the command line. That method always seemed to break for me.

Once this is done you can inject the chapters and rename to m4b all in a single command line. 

ffmpeg -i "ffmetadata.txt" -i "temp.m4a" -map_metadata 1 -codec copy "$artist - $albumSanitized.m4b"