iPod generation 4 won't upgrade ios
You can't update to iOS 9.  According to Apple, it's only compatible with iPod 5th &amp; 6th generation.

Apple stops signing (authorizing) previous iOS versions shortly after the new ones come out.  Because of that, you can't update to iOS 7 or 8 because they're no longer available from Apple.

You might be able to bypass this restriction by jailbreaking your device but I don't recommend that because there's many things that can wrong, including bricking your iPod.