These four words all introduce a subordinate clause, but each one means something different — picking the right one is about meaning, not grammar rules.

English "if" is the tricky one, because it can map to two different German words. Try replacing it with "whether" — if the sentence still makes sense, it's a question, so use ob (I don't know if/whether he's coming). If "whether" sounds wrong and it's really describing a condition (in that case...), use wenn (If it rains, I stay home — not "I don't know whether it rains").

All four also share one grammar rule: they push the conjugated verb to the very end of their clause (..., weil ich bin., not ..., weil bin ich.). That word order is drilled separately — this lesson is only about choosing the right word.