A friend of mine has had a badly cracked iPod 4 for a little while before i fixed it. Literally the whole screen was screwed to the point it was hard to use. Some parts of the glass fell out and you could even see some of the internal components. Nasty stuff I tell you! I decided to fix give it a try and fix it for him, without the apple price tag.
To be truthful, I don't know how i managed doing this at all. the screen was shattered into little pieces and was very difficult to get all the glass off without damaging the internal components. Then I had to make sure the glue was all off because a lot of the little glass pieces were still stuck to it. But i was eased when i finished getting the nasty screen off because I had some really easy instructions from ifixit's website. I just followed the procedure and almost everything went according to plan. Only issue that occurred was with the adhesive. on the left side of the iPod, i believe i didn't put the adhesive on correctly because that portion of the screen seems to bulge just a little bit, but I'm sure if my friend gets a good case to protect his iPod from cracking again, it should be no problem.
Good advice, hmm. Well first of all note that the entire front panel is basically glass and stuck the internal components. So, if you got a crack, that screen is gonna split and be difficult to take off, it should be no problem, but you gotta make sure no glass is left behind. Otherwise it might take up space in the components when you try to reinstall a new screen. Also when you get to the part when you put the new screen cable on the logic board, don't be afraid to pull at the logic board a bit to get as much space to put your fingers under it. I was really afraid to do it because i would not like having to replace a broken logic board as well. To be truthful, putting that cable on took a lot of time. Had to do it carefully to make sure i didn't bend anything or break anything.