If you have a large old-ish codebase you could be looking at a significant amount of work supporting 64 bit, so don't expect it to be a tickbox. Plus you may have headaches if third party libraries you use don't work in 64-bit.
Regarding the iOS8 SDK mentioned in the OP. It's a bit confusing, but when Apple talk about you using the iOS 8 SDK, it means that you must compile with up to date xcode. You can still compile with the iOS8 SDK but make it target iOS6 as its base SDK, so you don't necessarily need to replace your iPod4 because of the iOS8 SDK requirement. However, you do need to replace it so you can test your 64-bit compatibility.