The iOS 6 IPAs for this app are still hosted on the App Store. You are safe to buy it on another device and download the last compatible version on iOS 6. In fact, if you download aDowngrader from h6nry's repo, you can download this app on your iPod Touch 4g straight up without another device. Go to purchase the app, enter 13933597 when prompted and it will download. You will still have to pay.