I assume that the TRAF button on the 2004 acts the same way as on the classic Prius.
Typically, leaving the TRAF button on leads to more annoyances than it'll give you traffic reports. For example, you're listening to a cassette or CD, or just turn on the radio. Rather than returning to the FM station that you were last on, it goes to some station that you don't like (which usually isn't giving a traffic report).
I've only heard once of it actually working, and that report was from my mother (who has a 2002 Prius). For a few months, a FM radio station in her area (SE MA/SW RI) would actually provide traffic reports. She'd be listening to a cassette or usually a CD, and suddenly the music would stop, it would switch to this FM station, play the traffic report, and at the end of the traffic report it would go back to playing her music.
A few months later, they stopped issuing the traffic reports, and then she was stuck with the apparently random memory of last radio station issue (which I had to help her turn off the TRAF button to fix).