From the product page:
"With B4J you can easily create desktop applications (UI), console programs (non-UI) and server solutions.
The compiled apps can run on Windows, Mac, Linux and ARM boards (such as Raspberry Pi)."
Basically you create Java-apps (using the vb-syntax you are used to with B4A and B4i) which you can run on different operating systems. You can use it if you want to write an app for Windows 10 for example.
B4A: target is mobile apps for Android
B4i: target is mobile apps for iPhones and iPads
B4J: target is Desktop and Web apps for Windows, Mac and Linux (Raspberry Pi too)
B4R: target is apps for Arduino