BRUSSELS — The bomb maker, the transporter, the landlord and the cipher. The four men slipped away after the Nov. 13 attacks in Paris, and all but one reappeared as key figures in the Islamic State cell that went on to attack Brussels. Two are dead, one is captured, and the fate of the fourth remains a mystery. Who lives and who dies in an attack can provide crucial clues to how terrorist cells are structured, demonstrating who is considered disposable and who is crucial for the next job.