So, about a new board like UNO R4 WIFI, as usual you have to:
1) setup it in Arduino IDE, so it will be available in B4R after it
2) and goooogle-goooogle-gooogle about it
3) but all depends on the Arduino libs
Understand that B4R is just transpiller software over the Arduino - the language of higher layer that is just transcoded to C-code of MCUs, using Arduino's libraries.
Each MCU has its own development software kit (compiler, linker....) and this kit may be compatible with the Arduino IDE. B4R is one level up.