Replacing back case without tearing the button flex cable is extremely difficult since in the factory the cable is installed into the case first, then soldered onto the board after the board is installed. The ifixit guide made it backways because they tried to pull everything out. If you can do soldering, then refurbishing this device is much easier and straightforward.
I consider replacing the case of iPod touch 4 as complete refurbishment because it is simply too difficult to just replace the back case. Discard the old battery, copper adhesive and flex cable, buy new ones and putting them together is more worthy of the effort and actually easier.