I hope you enjoy your 4 S as much as I've enjoyed mine. I have iPhone and Android devices. The voice navigation of the Android is excellent. On my 4 S I have Motion-X ($0.99) It comes with a 30 day complimentary voice service. However, currently, they are offering a year subscription of $9.99.
Earlier today, I read an article that rated Waze as one of the best navigation apps. I've not tried it. It is a free app so check it out.
Let us know what you decide.
Mike