I've been developing an iOS application, and I've confirmed that it works fine on the iPhone 4. I have never tested it in a 4th generation iPod touch.
Recently I distributed the app to several testers (using a service called TestFlight). Only two of them are having problems downloading/installing, and they happen to be 4th generation iPod touch users.
So this leads me to my question – will an app that can run on the iPhone 4 be able to do so on a 4th generation iPod touch? Maybe their devices can't download/install the app because they actually can't run it.
The question at
http://store.apple.com/us/question/answers/ipod/will-all-the-iphone-apps-work-on-an-ipod-touch/QTH2T4XUYF29DAUUA says
Most apps will work on the iphone and the ipod touch but some apps
will require things that only the ipad or iphone has.
What are those specific things that an iPhone 4 may have that are lacking on a 4th generation iPod touch?
The app doesn't really do anything fancy. It uses Game Center, it has In-App purchase, an accelerometer… All of that seems to be supported by the iPod touch.
Best Answer
As a developer, you can control exactly which hardware you want to support and which you will not support by picking the source code you use and properly tagging the files you upload to Apple.
The iOS app store then can prevent the devices you have disallowed from downloading the app. So, there's nothing to prevent a developer from targeting only iPhone 4 with their code or just targeting a specific iOS version.
It's up to you (and each other developer) to decide on a build-by-build basis, so the general answer would be:
No, there is no guarantee that all apps (or any one app) that run(s) on iPhone 4 will also run on iPod Touch (4th generation).