How to create mp4 video at 6000kb/s in ffmpeg?

Question

fmpeg

I'm trying to compile a video in mp4 format. I specify the parameters for the bitrate, but this only works on the mpg format.
But when you select the mp4 format, the bitrate is reset to the lowest. How to assemble with a good bitrate?

exec('ffmpeg -f concat -safe 0 -i mylist.txt -c copy -b 6000k -minrate 6000k -maxrate 6000k output.mp4');