docs: add 4.0 upgrade section (#15343)

## Description

This change adds a section to the upgrade documentation about how to
upgrade from 3.0 to 4.0

(cherry picked from commit df81994)

Author: emmettbutler

docs/upgrading.rst

@@ -95,4 +95,20 @@ Use the following patterns to identify the deprecated environment variables in a
 Legacy tracing interfaces
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Reference the [3.0 release note](https://github.com/DataDog/dd-trace-py/releases/tag/v3.0.0) to identify and update deprecated interfaces in ddtrace products.
+Reference the [3.0 release note](https://github.com/DataDog/dd-trace-py/releases/tag/v3.0.0) to identify and update deprecated interfaces in ddtrace products.
+
+
+Upgrade to 4.0
+**************
+
+Environment variables
+^^^^^^^^^^^^^^^^^^^^^
+
+Use the following patterns to identify the deprecated environment variables in a code base. These configurations will not be supported in v3.0. Refer to the migration guide below for next steps::
+
+    git grep -P -e "DD_DYNAMIC_INSTRUMENTATION_UPLOAD_FLUSH_INTERVAL" \
+    -e "DD_EXCEPTION_DEBUGGING_ENABLED" \
+    -e "DD_PROFILING_STACK_V2_ENABLED" \
+    -e "DEFAULT_RUNTIME_METRICS_INTERVAL"
+
+Reference the [4.0 release note](https://github.com/DataDog/dd-trace-py/releases/tag/v4.0.0) to identify and update deprecated interfaces in ddtrace products.