The file the upgrade doesn't touch: `public/Application.cfc` from 4.0 to 4.1
Part 4 of "From 2.x to 4.x: the field guide" covers public/Application.cfc, the app-owned file that a vendor/wheels/ swap never updates. It replays every template change from 4.0.0 to 4.1.2 one commit at a time: the DI container, the onError fallbacks, the fail-closed reload gate, the X-Wheels-Reload-Password header, IP-based debug access, the session cookie, the Adobe teardown handlers, the jBCrypt load path and the .env parser. A table says what each edit fixes, which release added it and whether skipping it fails safe, and the post shows what changed on Lucee 7 and Adobe ColdFusion 2025 after each edit.