How do I mirror my iPhone screen to my MacBook Pro for free?
QuickTime Player can only mirror iOS devices with Lightning connectors.  If you want to mirror an iPhone 4s, you'll need to purchase one of the AirPlay Mirroring apps like AirServer.

According to this support article (emphasis added):


  Movie recording works with your built-in camera and many external cameras. If you're using OS X Yosemite or later, you can also record the screen of any iOS device (iPhone, iPad, iPod touch) with a Lightning port and iOS 8 or later.