In recent years, the Internet of Things (IoT) technology has propelled the widespread application of Wireless Medical Sensor Networks (WMSNs) in the healthcare field. WMSNs integrate various technologies to establish a remote diagnosis platform. However, the physiological data transmitted by WMSNs is highly sensitive, imposing stringent requirements on security and reliability. Currently, the mainstream three-factor authentication protocols have vulnerabilities. The reliance on third-party institutions brings risks of single-point failures and Distributed Denial of Service (DDoS) attacks, and there is also insufficient defense against physical layer threats. To address these issues, this study proposes a blockchain-enhanced four-factor authentication protocol. This protocol leverages blockchain technology to achieve decentralized authentication. Physical Unclonable Functions (PUFs) are employed to mitigate the security risks of sensor nodes and smart cards, while fuzzy extractors are utilized to safeguard the security of biometric templates. Through formal verification using the AVISPA tool and multi-dimensional informal analysis, it has been confirmed that this protocol can resist a variety of known attacks. Comparative analysis reveals that the proposed protocol not only attains a higher security level but also incurs the lowest communication overhead, making it highly suitable for the resource-constrained environment of WMSNs.