Upgrade to v4 docs

Author: ericallam

docs/upgrade-to-v4.mdx

@@ -713,5 +713,16 @@ We recommend the following steps to migrate to the new run engine:
 
 1. Upgrade to the v4 package.
 2. Run the `trigger dev` CLI command and test your tasks locally, fixing any breaking changes.
-3. Deploy to the staging environment and test your tasks in staging, fixing any breaking changes.
-4. Once you've verified that the new run engine is working as expected, you can deploy to the production environment.
+3. Deploy to the staging environment and test your tasks in staging, fixing any breaking changes. (this step is optional, but recommended)
+4. Once you've verified that the new run engine is working as expected, you should deploy your application backend with the updated v4 package.
+5. Once you've deployed your application backend, you should deploy your tasks to the production environment.
+
+Note that between steps 4 and 5, runs triggered with the v4 package will continue using the previous engine, and only new runs triggered after step 5 is complete will use the new engine.
+
+<Warning>
+  Once the new run engine is activated, there will be a period of time where old runs will continue
+  to execute using the previous engine, while new runs will use the new engine. Because these
+  engines use completely different underlying queues and concurrency models, it's possible you may
+  have up to double the amount of concurrently executing runs. Once the runs drain from the old run
+  engine, the concurrency will return to normal.
+</Warning>