Spring Boot 4 introduces a fundamental architectural change by splitting the monolithic spring-boot-autoconfigure jar (grown from 182 KiB to 2 MiB) into smaller, focused modules. This modularization improves maintainability, reduces classpath overhead, eliminates unnecessary auto-configuration, and enables new use-cases like using Micrometer metrics independently of Actuator. Migration involves refining starter dependencies, using new test starters, adjusting package imports, and optionally leveraging "classic starters" for gradual transitions. Each technology now has its own dedicated starter and module with clear boundaries.