I don't think it's compatible with B4X. For Rasberry Erel suggest to use B4J , since normally it uses a distro of Linux and it contains a Java runtime. But this board supports no OS, so...
This is correct. But since it can be programmed with C++ as it is indicated in its site,
@Erel already has knowledge of C++, and possibly he can create the "translation" from B4R to Raspberry Pico C++. Further more the Pico cannot run java since it is a microcontroller and not a one board computer. If
@Erel can manage to create this "translation" then it is a matter of how easy it is to cooperate with the Raspberry IDE to compile and upload "by pass".
Edit: The most suitable case would be that the Arduino Environment would "adapt" the Pico and create themselves the translation, compilation and upload. But since they compete each other, this, is in the "dream" world.