I bought an InvisibleSHIELD for my new iPod. It has a manufacturing defect, there is a great big rainbow colour scratch running vertically down it, but that's another story.
I found it quite easy, though time consuming, to apply. Make sure your hands are CLEAN, and there is *no* dust on your device. Use that micro fibre cloth!! I recommend doing the back first. You should also watch an install video, the 1st gen iPod one is pretty similar to the 4th gen.
Spray the adhesive side liberally, place it down on the back, slide it around and LINE IT UP perfectly with your fingers, spray the non-adhesive side, and use the squeegee liberally to push out all the bubbles. I found the side with the bit that sticks out works best.
You *will* have to push hard to it to get the micro bubbles out, however, what they say is true - they do work themselves out after 2-3 days. Mine started with many left on it, now there are just 3!
Make sure you "palm" the edges, and hold it for a good 15-20 sec. Sticks fine once you do that. Squeegee to bubbles out once again.
As for the front, don't make the mistake I did. Peel off the main screen (front camera part with it), spray, lay it down, and line it up (this is really easy on the screen, and you'll need it to be dead perfect.
Now, take the OUTER RING of the Home Button and stick it down, THEN the main Home Button. The outer ring will probably come with the Home Button, but you need to separate it! Otherwise, you will end up with a squishy, non-clicky, unresponsive Home Button.
Hope all that helps you out!!
As for cases,I've found it fine with the cheap case I have. It's adds all of 0.2mm to the device, so it shouldn't be a problem with silicon or TPU/soft plastic cases. However, hard plastic or metallic shells, you may have problems with, as they need to be tight to remain secure - 0.2mm added thickness may make or break the case fitting.
Good luck!
EDIT: Also, make sure you line up the microphone hole & camera on the back - preferably even start there. You don't want no sound on your videos!! Make sure you're careful around openings to not get them wet, but if you do don't worry - a paper towel quickly fixes that. Make sure you dry it quickly though.
Oh, and finally, make sure you've got at least half an hour spare time, work in a clean, dust-free area, and LET IT CURE OVERNIGHT! Don't touch it until the next morning!!