4.0.0 will GA the Java Performance API. That means both Android and Spring will have it, but spring includes also auto instrumentation.

Java main API:

• Add environment and release to all transactions environment and other fields are missing for transactions #1147 (PR: Fix: Set release and environment on Transactions. #1152)
• Delete the preview note (PR: Java SDK 4.0 Docs update sentry-docs#2833)
• Document connecting errors and spans (PR: Java SDK 4.0 Docs update sentry-docs#2833)
• Document changes in the transport (the new apache transport ApacheHttpClientTransportFactory auto-configuration. #1143) (PR: Java SDK 4.0 Docs update sentry-docs#2833)
• Document changes in the API once (Polish Performance API #1165) and (Fix potential NPE by separating transaction contexts. #1164) are merged, Fix Java/Kotlin code snippets for Performance API sentry-docs#2912
• traces-sampler-as-sampler is different for Java and Kotlin snippets traces-sampler-as-sampler is different for Java and Kotlin snippets sentry-docs#2850

Android:

• Alpha: Add docs for manual instrumentation (Make sure Kotlin samples are there) Performance docs for Android sentry-docs#2851
• Alpha: Verify it works properly with offline caching (always-on on Android)
• On the samples in this repo, opt-in for performance with random sampling 1.0.

Spring:

• New Transport for Java backend: Java Server Transport #1097 (PR: High throughput transport implementation #1114, Add non blocking Apache HttpClient 5 based Transport. #1136)
• Docs to point users to opt-in to the Apache transport (PR: Java SDK 4.0 Docs update sentry-docs#2833)
• Add support to datasource-proxy and documentation (PR: Database queries tracing with datasource-proxy. #1095)
• Performance for Spring (non-boot) (PR Simplify Spring integration #1188)
• Performance docs for Spring instrumentation (currently only Spring Boot has it)

Discussions

• Clarify differences in API to align with .NET (and help build Flutter and iOS that come next): Performance API follow up sentry-dotnet#663 (PR: Polish Performance API #1165)
• Address the developer docs points: Improve transaction & span docs develop#221
• Scope's transaction field Document Scope's transaction field develop#246
• API suggestion for finishing a transaction/span API suggestion for finishing a transaction/span develop#245
• Contexts.trace Document Contexts.trace develop#244
• Sampling decision Document where/when the Sample decision should be made develop#243
• Document default values for 'Span.op' on auto instrumented SDKs Document default values for 'Span.op' on auto instrumented SDKs develop#247
• TracesSampler returns null (idea from .NOT) TracesSamplerCallback allows returning a nullable Double #1204