Dojo

Tatami is a fighting arena in Dead or Alive 4, which is only avaliable during Sparring Mode, though it can be unlocked for other modes by doing some tasks. It reappeared later in Dead or Alive: Dimensions under the name Dojo. In Dead or Alive 5, the appearance looks the same.

As the name suggests, it is simply an endless ground covered in, with no walls or other hazards in the way. This is the best location to practice moves or complete exercise mode, as there is an endless room to work in and there is nothing in the way to ruin long moves or combos.

Unlocking for VS Mode in Dead or Alive 4
The Tatami stage can be unlocked outside Sparring Mode by unlocking all costumes (by completing Story Mode or Time Attack Mode with default settings- one costume per time) and unlocking all system voices (by completing each character's excercise).

You need to do these for the unlockable characters too (Time Attack Mode for costumes), though it should be noted that Gohyakumine Bankotsubo has no unlockable outfits, but you still need to do his exercise.

Also note a certain throw called the Raijin: it can ruin your chances of unlocking the Tatami for the other modes. It is part of Hayate's exercise, and the hardest throw to do in the game. However, it has a simple three step sequence, and once you get the hang of it, it can be quite easy to do.