"Scientists are still trying to" is correct.
"Understand how the firestorms" is correct.
"Are created although weather, terrain, vegetation" is also correct.
"And characteristics of the fire itself play a role" contains an error in structure. The conjunction "and" should not begin the sentence. A comma before "and" would improve clarity.
Thus, the error is in option (4), where the sentence would be better with a comma before "and."