iPod touch (4th generation) is not compatible with iOS 7. Unfortunately the hardware just won't support it. You may consider getting a new iPod touch (5th generation) which will be compatible with iOS 7 when it is released this fall. iPod touch (5th generation) starts at $229.
Unfortunately, you cannot get the iOS 7 on iPod Touch 4th Generation. This is because this app is not yet applicable to this device and according to Apple, only the 5th Generation is compatible. Check out http://www.apple.com/ios/ios7/features/.