diff --git a/4.0-Upgrade.md b/4.0-Upgrade.md
index 1a35f0bb..b5c2e1bd 100644
--- a/4.0-Upgrade.md
+++ b/4.0-Upgrade.md
@@ -25,6 +25,9 @@ gem 'redis-namespace'
   `concurrency + 2` connections in your pool or Sidekiq will exit.
   When in doubt, let Sidekiq size the connection pool for you.
 
+* Worker data is no longer updated in real-time but rather upon every
+  heartbeat.  Don't expect the `Sidekiq::Workers` API to be millisecond-precise.
+
 * There's a new testing API based off the `Sidekiq::Queues` namespace. All
   assertions made against the Worker class still work as expected.
 ```ruby