I replaced a battery in an iPod Touch 4th generation this afternoon. It is not the easiest of tasks, especially as I had not replaced one before. It is now working again and I currently have it charging up ready to return to the owner. One tip I would add to the YouTube videos showing you how to do this, is to use a hair dryer to warm up and soften the glue around the shield covering the battery after removing the screws. I found it easier to prise away from the circuit board without having to apply too much pressure and risk damaging the circuit board. Applying a little heat also made it easier to lift the battery away from the copper foil heatsink. Someone appeared to have been very generous with the glue during manufacture.
Oh, and one more thing. Those of us of a slightly older generation attempting this repair will find a large magnifying glass comes in very handy.