Cannot resolve symbol v4

The "Cannot resolve symbol v4" error typically occurs when you are referencing the Android Support Library v4 but have not properly imported it in your code.

To resolve this issue, you can do the following:

1. Make sure that you have added the support library to your project's dependencies. You can do this by adding the following line to your project's build.gradle file:

implementation 'com.android.support:support-v4:28.0.0'

Note that the version number may vary depending on your project setup.

2. After adding the dependency, sync your project with Gradle by clicking on the "Sync Now" button or running the Gradle sync command.

3. If you have already added the dependency and are still facing the issue, try cleaning and rebuilding your project. This can be done by selecting "Build" from the top menu and then choosing "Clean Project" or "Rebuild Project".

4. If none of the above steps work, make sure that you have the necessary Android Support Library v4 files downloaded in your SDK Manager. You can open the SDK Manager by clicking on the SDK Manager icon in your Android Studio toolbar.

By following these steps, you should be able to resolve the "Cannot resolve symbol v4" error and successfully reference the Android Support Library v4 in your code.