I am not sure how FreeBSD got involved, but the following statements are true
1) Raspbian Buster is the only released version of Raspbian that runs on the RPi 4
2) Mathematica 12.0 for Linux-ARM is compatible with Raspbian Buster
3) Mathematica 12.0 for Linux-ARM is compatible with Raspbian Stretch
4) Mathematica 11.3 for Linux-ARM is not compatible with Raspbian Buster
5) Mathematica 11.3 for Linux-ARM is compatible with Raspbian Stretch
6) RPi 3 or earlier may run Raspbian Buster or Raspbian Stretch