diff --git a/TODO.md b/TODO.md
index 2c71e18..d55b1a8 100644
--- a/TODO.md
+++ b/TODO.md
@@ -24,6 +24,7 @@ Replace our current scripts with Bitcoin Standup (which was based on those scrip
    * <strike>Re-edit Chapter 2</strike> **6/19**
    * <strike>Edit & Check Chapter 3</strike> **6/19**
    * Edit & Check Chapter 4
+      * Double-check fee calculator in 4.2I with a more complex example
    * Edit & Check Chapter 5
    * Edit & Check Chapter 6