How many maturity levels (ML) are established for evaluation criteria according to ISA/IEC 62443-2-4?

ISA/IEC 62443-2-4 defines four Maturity Levels (ML1 to ML4) for evaluating the processes and practices of service providers (such as integrators and maintenance organizations). These maturity levels assess the consistency, documentation, and effectiveness of processes, ranging from “Initial” (ML1) to “Improving” (ML4).