What are the special requirements for designing SIL 4 systems?

SIL 4 system requirements: PFDavg < 10^-4 to 10^-5 (extremely low failure probability), hardware fault tolerance of 2 minimum (triple redundancy), diverse redundancy to address common cause (different technologies, manufacturers, or measurement principles), very high systematic capability (rigorous development lifecycle, independent assessment), dedicated safety systems separate from control, extreme diagnostic coverage, frequent proof testing or continuous self-diagnostics. SIL 4 is rarely used in process industry (IEC 61511 generally limits to SIL 3) - more common in nuclear, railway, and aerospace. Requires certified logic solvers, extensive verification, and independent functional safety assessment. Cost and complexity usually justify alternative approaches (inherently safer design, reduced demand rate).