Well im pretty sure our N900 can produce beyond 4Khz as you stated it was for smart-phones in general, looking at the speakers i know that they can go way beyond that, an take it from someone that been building HiFi for over 10 years, and have also made a very small speaker set made by the very speakers that's in our N900/N95

But yes on the dB level, that's why i would say a dog whistle is a better choice, it hits the high note (that we can't hear) and like a normal whistle it does generate some dB


EDIT: Not saying that this is impossible tho