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If you hit a 1604, 1603, 1602, 1601, 1600 error while trying to restore your iPhone with a custom iOS 4.2.1 firmware file created using Sn0wbreeze 2.2 for Windows or PwnageTool 4.2 for Mac then there is a way to bypass the error.
iH8sn0w, developer of Sn0wbreeze has released a new version of iREB tool, which is compatible with iOS 4.2.1.
iREB is a tool that will put your device into a pwned state so that you can upload custom firmware to it.
iREB RC3 supports the following iOS devices: