弹出viewcontroller然后回到RootViewController似乎不起作用

I have an app that pushes four viewcontrollers on the stack. In the fourth view controller, I do a PopViewController, so I should be back at the 3rd viewcontroller, and in the viewWillAppear method, I do a PopToRootViewController. This does not get me back to the first viewcontroller correctly. The code is straightforward, just a I have an app that pushes four viewcontrollers