diff --git a/README.md b/README.md
index 4a96a3f..5248e69 100644
--- a/README.md
+++ b/README.md
@@ -101,6 +101,16 @@ def handler(event, context):
     return "Hello World!"
 ```
 
+### Structured (JSON) logging
+
+When the function is configured to emit JSON-formatted logs (`AWS_LAMBDA_LOG_FORMAT=JSON`), logs written by the runtime for uncaught errors include the log level under the `level` key, e.g.:
+
+```json
+{"timestamp": "...", "level": "ERROR", "errorType": "...", "errorMessage": "...", "stackTrace": ["..."]}
+```
+
+If you parse these logs, read the level from `level` (this field was previously emitted as `log_level`).
+
 ### Local Testing
 
 To make it easy to locally test Lambda functions packaged as container images we open-sourced a lightweight web-server, Lambda Runtime Interface Emulator (RIE), which allows your function packaged as a container image to accept HTTP requests. You can install the [AWS Lambda Runtime Interface Emulator](https://github.com/aws/aws-lambda-runtime-interface-emulator) on your local machine to test your function. Then when you run the image function, you set the entrypoint to be the emulator.
diff --git a/RELEASE.CHANGELOG.md b/RELEASE.CHANGELOG.md
index a3fe32c..153556f 100644
--- a/RELEASE.CHANGELOG.md
+++ b/RELEASE.CHANGELOG.md
@@ -1,3 +1,7 @@
+### September 15, 2026
+`4.0.4`
+- Use the `level` key (instead of `log_level`) for the log level field in JSON-formatted uncaught error logs, aligning it with the key used by other structured log events ([#221](https://github.com/aws/aws-lambda-python-runtime-interface-client/pull/221))
+
 ### September 2, 2026
 `4.0.3`
 - Emit a structured `runtime_worker_pool_initializing` DEBUG log event once per execution environment during INIT in multi-concurrent (Lambda Managed Instances) mode, reporting `workerCount` and `executionEnvironmentMaxConcurrency` for worker pool observability. Only visible when the function log level is DEBUG or lower; no impact on the standard on-demand path.
diff --git a/awslambdaric/__init__.py b/awslambdaric/__init__.py
index f00368b..b479cf6 100644
--- a/awslambdaric/__init__.py
+++ b/awslambdaric/__init__.py
@@ -2,4 +2,4 @@
 Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 """
 
-__version__ = "4.0.3"
+__version__ = "4.0.4"