The fourth installment in a series on deploying applications with Kubernetes using infrastructure as code focuses on stateful applications, such as databases, and how they differ from stateless ones by requiring persistent storage, which poses unique scaling and availability challenges. In Kubernetes, StatefulSets are used to manage stateful applications, providing stable and unique network identifiers and persistent storage, essential for applications needing ordered deployment and updates. The article illustrates this with a MariaDB deployment example, highlighting the use of StatefulSets to maintain persistent identifiers for pods, Headless Services to assign direct IP addresses, and volumeClaimTemplates to manage storage access. It further discusses configuring health checks like liveness and readiness probes to ensure pod reliability and explores pod scheduling rules to manage deployment locations, ensuring that resources maintain stable connections. The article concludes by emphasizing the importance of Kubernetes' StatefulSets in routing traffic to stable resources and encourages readers to explore the full example on GitHub and further their Kubernetes knowledge with suggested resources.