If an app runs on the iPhone 4, will it run on the 4th generation iPod touch?
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).