IPhone 4 boot loop, no DFU possible

To enter DFU mode:

1. Make sure the phone is on and iOS fully loaded, and iTunes is closed.
2. Plug the phone into your computer using the Apple proprietary cable.
3. Once plugged in, turn your phone off by holding the power button.

4. When the phone is completely powered down, perform the following sequence:

   1. Hold the power button for 3 secs.
   2. Without letting go of the power button, press and hold the home button for 10 secs.
   3. Without letting go of the home button, release the power button for another 10 secs.

Ignore what's happening on your screen, just time the sequence. If you are not succeeding, then you are not timing the sequence properly. There is only one way to enter the device into DFU mode. The one provided by Redsn0w is the correct one.

This may seem silly, but this is what I do when I have had to troubleshoot DFU mode (getting in and out of). First cross your fingers that it is software. Do you have a different USB cable that you can try with? Access to another computer to attempt with. I'm sure you have already tried rebooting both the iPhone and Mac, right? Not sure if you know or not, but you actually don't the computer nor the cable to get in DFU. You may want to try without plugging it in as well. The only difficult part with that is since it is not plugged in you will not know when it boots into DFU mode until you plug it in and the computer chimes. These seem basic but even the best of us forget about switching ports, rebooting the computer or trying another cable.