Keep in mind you have to have itunes 11.1.5.5 or earlier for redsn0w to work on your PC.

It will then walk you through the rest of the process. To use Redsn0w: download latest version here We will be using iDeviceRestore to restore the IPSW, but sometimes the pwndfu function built in doesn’t work, that’s why I mention other methods here:

To do this, put your iPhone into DFU mode (hold power and home for 10 seconds, then continue holding home button until you iPhone is recognized by your computer), then use iReb, iDeviceRestore, or Redsn0w on a PC (**this method only works on Windows 8.1 and older at the moment because windows 10 does not work with Limera1n at the moment**), or ipwndfu or iDeviceRestore on Mac to put it in pwndfu mode. To restore these bypassed IPSW’s you will need to put your iPhone into pwndfu mode, which is a state of DFU mode that bypasses signature checks. If you don’t know, you can connect to 3uTools and it will tell you, or you can just try all three IPSW files and see which one restores. Download my bypassed IPSW’s according to which model of the iPhone 4 you have.