Sorry to hear about his iPod. Unfortunately, you can't use any previous generation screens for your 4th generation iPod touch. It uses the same type of display as the iPhone 4 (not the exact same part, though), so the glass, digitizer, and LCD are all one inseparable piece.
If you find the part somewhere, all you would have to do is find a decent disassembly or repair guide (our [guide|3562|teardown] might provide a good starting point). As you can see from step 15 there, you have to remove the entire logic board before you can take out the display assembly.
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== Update ==
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Since posting this we have created a [http://www.ifixit.com/Guide/Repair/Installing-iPod-Touch-4th-Generation-Front-Panel-Replacement/3639/1|front panel replacement guide] for the 4th Gen. iPod Touch. This instructions will be much more helpful than the Teardown. Unfortunately, we still don't sell the part, but we do sell the necessary tools.
Sorry to hear about his iPod. Unfortunately, you can't use any previous generation screens for your 4th generation iPod touch. It uses the same type of display as the iPhone 4 (not the exact same part, though), so the glass, digitizer, and LCD are all one inseparable piece.
If you find the part somewhere, all you would have to do is find a decent disassembly or repair guide (our [guide|3562|teardown] might provide a good starting point). As you can see from step 15 there, you have to remove the entire logic board before you can take out the display assembly.