FBSOpenApplicationErrorDomain code =4 error

Just closing the iOS Simulator worked for me.

There is not always the need to reset the iOS Simulator. I did not even had to close Xcode.

This happened to me once when I added some assets as symbolic links (through ln -sf). Even installing to devices was failing.

After I removed the links and made a hard copy, it worked and error was gone.

No need to quit the simulator or reset all content every time .

Just close the app on which you are working if running on background .

it works for me .

This worked for me:

iOS Simulator -> Reset Contents and Settings... -> Reset

referenced from Qiita: Unable to run app in Simulator エラーの対応方法.