I have a played a lot of Tekken in my time. Between my introduction to the series with Tekken 2 and the occasional romp on Tekken Tag Tournament, I've probably logged over 100 hours with the series, which is impressive when you consider I usually don't like fighting games. With each new Tekken that comes out, I tell myself I've had enough, that I've already played these games way too much, but Namco has always pulled me back in with new characters, new mechanics, or even new graphics that make it a fresh experience. This just wasn't the case with Tekken 4.
As Brad said, the game's core engine is relatively untouched. Most of the characters are back from old Tekkens with only slight modifications (if there are any at all). The new characters are either exact copies of classic characters or boring stereotypical additions (another big strong, slow guy or a quick young kickboxer. Yawn). Instead of true innovation, we get breakable walls that have (1) been in series like Dead Or Alive for years and (2) actually detract from the experience of the game, turning each match into a battle of who can push the other into the corner first.
That's not to say the game is totally without merit. Like Brad, I enjoyed the expanded characterization in the story mode, and the totally redone Jin was a pleasant surprise. But the reduced character lineup and lack of Namco-standard extras make it seem like less of a game than its predecessor, which is something that should never be said about a sequel.
Basically, there's nothing here to attract anyone who already owns Tekken 3 or Tekken Tag Tournament. It's half a step forward and two steps back for a series that has already been done to death. Next time, Namco will have to give me something new and worthwhile if they want my $50.