Spring Boot 4.0 introduces significant improvements including better modularization for reduced memory usage, JSpecify as the standard null-safety library, and enhanced observability with Micrometer. The release establishes Kotlin 2.2 as the official baseline, bringing native JSpecify annotation support, cleaner API versioning, and more elegant bean registration through Kotlin DSL. IntelliJ IDEA 2025.3 provides full support for these features with compiler warnings and IDE inspections for null-safety violations.