iPhone developer and hacker iH8sn0w has announced the release of iREB RC4. This is the new updated version of iREB which comes with new improvements and fixes some issues related to the Classic Theme and first generation iPod Touch. Now you can use this app on both iOS 3.x and iOS 4.x. For your kind information using iREB you can load custom firmware on your iOS device by putting it in a pwned DFU state. You can use this app to kick your iOS device out of recovery mode.
Click here to download iREB RC4 for Windows. This app also allows you to bypass 1604, 1603, 1602, 1601, 1600 error in iTunes during custom iOS 4.2.1 firmware restore. You can follow the step by step instructions posted here to fix 1604, 1601, 1600, 16xx Restore Error in iTunes with iREB RC3.