In order to replace the screen on your 4th generation iPod Touch you will need to partially remove the main logic board. This is an advanced level repair and may result to permanent damage to your iPod. Be careful if you decide to perform this repair.
In this video I will demonstrate the replacement of an iPod Touch 4th generation LCD. The replacement part can be ordered through the link below.
One issue that people tend to encounter during the installation of the new LCD is failure of the screen to properly adhere to the frame. This can be caused by several different things including improper alignment, a flex cable which is not folded properly, bent corners on the housing and forgetting to transfer the metal clip at the upper end of the lens. If you have checked to be sure that all of the above are not the reason that your screen will not adhere properly it is sometimes possible to add a small amount of glue to the corners of the iPod frame in order to hold the lens in place. This should only be used as a last resort when one of the corners especially the top left hand side lift off of the frame over time. I recommend using crazy glue or the type used for attaching acrylic nails. You can clean up and remove any excess with acetone. Be careful when using glue on your iPod as too much can create additional problems.