add breaking change docs

Author: siriwatknp

docs/src/pages/guides/migration-v4/migration-v4.md

@@ -1124,6 +1124,8 @@ You can use the [`collapse-rename-collapsedheight` codemod](https://github.com/m
   +<Radio color="secondary />
   ```
 
+- The className does not have `.MuiIconButton-root` and `.MuiIconButton-label` anymore, target `.MuiButtonBase-root` instead.
+
 ### Rating
 
 - Move the component from the lab to the core. The component is now stable.
@@ -1358,6 +1360,8 @@ You can use the [`collapse-rename-collapsedheight` codemod](https://github.com/m
   +<Switch color="secondary" />
   ```
 
+- The className does not have `.MuiIconButton-root` and `.MuiIconButton-label` anymore, target `.MuiButtonBase-root` instead.
+
 ### Table
 
 - The customization of the table pagination's actions labels must be done with the `getItemAriaLabel` prop. This increases consistency with the `Pagination` component.