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Did you go to the app store from the 5s to download iWork?

It is free for newly purchased devices. Once you get it from the 5s it should then be available for download on the 4s in the App Store app under Updates > Purchsed

You don't get it by simply updating to iOS 7. It is only available on new devices.

Open the App Store from your new iPhone. If you don't get a prompt look for them in the App Store and they should be free. If not contact iTunes Store support. They will issue you a redemption code.

http://www.apple.com/support/itunes/contact/