Hard Reset SONY Xperia Z4

• Start by turning on the device by holding the Power key for a short while.
• In the next step find and select Settings and tap Backup & Reset.
• Afterwards tap Factory Data Reset and then Reset Phone. 
• Choose Erase Everything if you are sure you want to clear all your data. 
• Next, select OK to confirm the whole operation.
• Well done! The hard reset has just been performed.

Seconds method:

• First of all, you should download PC Companion, in example from here: http://www.sonymobile.com/gb/tools/pc-companion/
• Next, install and run this application.
• Then you will see window with 4 options.
• Choose option "Support Zone New!", and click "Start".
• Wait for the program to install the appropriate add-on.
• After that you will see next window with 3 options.
• Go to option "Phone / Tablet Software Update", and click "Start".
• Select "repair my phone / tablet".
• Now follow the instructions and accept all informations.
  
• Please be patient and wait until preparing computer for software update is complete.
• Afterwards choose your phone / tablet model.
• If you see this window, then connect your device into your PC.
• That's all

Third method:

• At first, download the flashtool from Sony website: http://developer.sonymobile.com/services/flash-tool/
• Make sure, that you have installed drivers for your USB data cable.
• After that, power off your device
• Then press and hold Volume Down, and connect to USB cable.
• Follow the instructions on your screen, and accept appropriate options.
• Success! Your phone is ready to use.