Sometimes syntax highlighting does not work on Xcode 4. Syntax highlighting is an important feature to developers. But there is no "Rebuild Code Sense" menu on Xcode 4. We can do it with following steps:
1. Xcode menu: Window->Organizer
2. Go to Projects tab
3. Choose your project on the left panel.
4. Click "Delete…" button after the Derived Data on the right panel.
Xcode will start to rebuild code sense of your project.