Tekken 4 doesn't really move on from Tekken 3, and while there are secrets, they are few and easy to come by.
In short, not a lot. In fact all it really does is bump it to semi next gen level compared to Tekken Tag. There are good things, such as the improvement in visuals. Characters appear much smoother than before, and in fact look more human/robotic, whatever you play as, more even more flowed than before. The stages are also more beautiful looking in some places, and interaction with stages although limited, is a nice advancement in fighting games.
There are also a wider variety of fighting styles. With the return of some characters, others have changed how they are played. Jin Kazama in theory would have the same moves as Kazuya Mishima (reintroduced in this one), but in a nice little plot development, he's unlearned the Mishima style and taken up Karate, making him effectively a brand new character. A few others are new, such as Christie Monterio (female Eddy Gordo), Steve Fox, a boxer who can't kick, Craig Marduk, a Vale Tudo beef-cake and Combot, pretty much like Mokujin from the last game. There is another one, but he's actually a previous fighter.
In addition to the fighting, you also have the typical Time Attack, Survival and Practice modes. There is also a Training mode and Tekken Force mode. The latter is a side scrolling beat-em-up much like the last Tekken Force, only this is better designed. So is there an actual point to these except for bragging rights? No... And thats where the problems start.
The amounts of secrets reach about as far as characters, and all of them can be unlocked by completing Story Mode various times. There are a few secret characters, including said Jin Kazama and Combot, but apart from that there are no others. This makes the insentive to play this game not last too long. Also even though there are difficulty settings, the game overall on Normal Difficulty is not hard at all, in fact this is the easiest fighter in years, even the last boss is ridiculously easy.
The other problem lies with the characters not being as smooth as they could have been, and some moves being really hard to execute compared to other Tekken games. So when you lose a fight, it might not be because of the fact the computer whomped you badly, but your fighter not doing what you want it to do. Problems seriously lie with crouching and side-stepping.
Multiplayer is limited to fighting. Which is satisfactory, just. There is also a theatre mode where you can review tracks, prologues and epilogues. But all of this still doesn't quite make it a complete package, and thats the problem with this game in particular.
So a fairly solid but buggy fighter, with characters that are easy to unlock, and not much insentive to do anything else leaves this game coming up short. If you are a hardcore Tekken fanatic then this won't disappoint, but anyone looking for something with lots to achieve, then go with something else.