ITIL (Information Technology Infrastructure Library) is one of the most widely adopted frameworks for IT service management, offering structured practices to enhance service quality, efficiency, and governance. However, existing maturity assessment models are outdated and not aligned with ITIL 4, limiting their practical application. This study proposes a structured and practical maturity assessment model tailored to ITIL 4, incorporating elements from the Process Maturity Framework (PMF) and Capability Maturity Model Integration (CMMI 2.0). Developed through the Design Science Research (DSR) methodology, the model was applied in the Information and Communication Technology (ICT) department of a Brazilian public agency over two assessment cycles. It enabled the diagnosis of maturity levels, identification of service management gaps, and implementation of targeted improvements. Results include a reduction in initial-level practices (from 41 to 26%) and advances in practices such as Change Enablement and Service Request Management. The model also improved team integration and strategic alignment. Although tested in a single organization, it demonstrated adaptability to other contexts. The proposed framework offers ICT departments a robust tool for guiding continuous improvement and aligning operations with ITIL 4 principles.