I dont understand why you feel it's nessesary to make the mp4 to mp3 first
There are CD players that are capable of playing what are known as "MP3 disks" which are just CD-Rs that have MP3 files burned on them, or USB flash disks with MP3 files on them. gottscheer wants to create MP3 CD-R disks.
There are some (relatively few) newer CD players that are also capable of playing MP4 audio files, but I haven't seen many of these around.
You have to check the specs on your specific CD player to verify what kind of disks and files it can play.
any selfrespecing burning programs should be able to burn mp4 into cd digital audio as easy as burning mp3 into digital audio
Yes, but this creates a "playable" CD audio disk, which has its own unique format and the tracks are encoded as AIFF files. This is how commercial CD disks are encoded & burned. It's not the same as an MP3 disk.