B4A is for developing apps for Android devices .. and I believe this is the reason you purchased it.
B4J you can create Java based programs / apps for use on desktop /PC's
They are two different animals .. but the coding is similar in nature. Have you had any previous experience with Visual Basic .? .. think B4J.
You will find a few more resent tutorials / videos will be presented in B4J ... but with some minor tweaking the presented code still works with the other two products.
You do not need B4J at all to develop a Android app... it was only a suggestion , as the software is offered free.
Just this weekend I have taken a completed B4A Android phone app and exported everything to my PC (b4J ) and changing / tweaking some code differences
I nearly have an identical PC Desktop app to the tablet version .. wich will eventually share data via cloud database.
Hope this helps..
PS ... have you seen /read Beginners guide
https://www.b4x.com/android/files/guide.zip