I raise such problem but in the same post I raise the solution.
The first thing to know is that the .camrec format (from Camtasia) is a compressed file, so we must first decompress these files before converting them to .mp4.
Using 7z it is possible to decompress a .camrec file, and generate a .avi:
7z e "*file.camrec" "file.avi"
The second thing, is to convert this .avi file to .mp4 with ffmpeg.
ffmpeg -i "file.avi" "file.mp4"
If someone has a better solution I invite you to post it here.