iMessage is a new software by apple that came out with iOS 5. You need to download iOS 5 on your iPhone 4 before you can text your daughter on her iTouch. You can do this by connecting to iTunes with your iPhone and checking for any updates, and it should come up. After that, you'll be able to message your daughter through your iPhone.
With the iPhone, create a contact for the person with the iTouch. There will not be a phone number to enter like you normally do, but enter in the Apple ID for the iTouch user under the e-mail section of the contact.
When you go to message from iPhone to iTouch, just type in the contact name and it should message the iTouch via Apple ID.