Ein:
To unlock Ein in other modes, play through Story mode as Hitomi and defeat him at the end. After the intermission sequence you will be notified that Ein is unlocked. This applies to all modes but Story mode, as hidden characters do not have a story.

Gen Fu:
Play through the game as Eliot and finish his Story mode. After the intermission sequence you will be notified Gen Fu is now playable in all modes except Story mode. No hidden characters can be used in Story mode.
User Cheat by DrunkenMaster

Helena:
Complete Story mode with all sixteen characters to unlock her in all modes.

Leon:
Complete Story mode as Zack to unlock him in all modes except for Story mode.

Spartan-458:
Complete Story mode as Helena to unlock Spartan-458 from Halo in all modes except for Story mode.

Tengu:
Complete Time Attack mode with all characters, including the bonus characters.

Halo stage:
Complete Story mode as Helena to unlock a stage similar to the first stage of Halo 2, the Hangar Area.

Increased bounciness:
As in the previous Dead Or Alive games, you may enter your age as "99" by going into your profile from the main menu. This will result in the female characters' breasts to bounce more than usual.
User Cheat by Niknasty

Alternate costumes:
Complete Story mode to unlock an alternate costume with the following characters. If available, the next alternate costume for that character will be unlocked when Story mode is completed again with them.

Ayane C3, C4, C5, C6
Bass C3, C4
Bayman C3
Brad Wong C3
Christie C3, C4
Elliot C3, C4
Hayabusa C3, C4
Hayate C3, C4
Helena C3, C4 C5
Hitomi C3, C4, C5, C6, C7, C8
Jann Lee C3, C4
Kasumi C3, C4, C5, C6, C7
Kokoro C3, C4, C5, C6, C7
La Mariposa C3, C4, C5
Leifang C3, C4, C5, C6, C7
Tina C3, C4, C5
Zack C3, C4
Complete Time Attack mode under the default settings to unlock an alternate costume with the following characters. If available, the next alternate costume for that character will be unlocked when Time Attack mode is completed again with them.

Ein C3, C4
Genfu C3, C4
Leon C3, C4
Spartan-458 C3, C4, C5, C6, C7
Change system announcer's voice:
In the options menu, notice that you can choose different voices for the system announcer. To unlock additional voices, enter sparring mode and bring up the sparring menu. Select "Exercise" and you will be prompted to run through an extensive moves list. Once you complete it, that character's voice will be available as a system announcer.

Get easy costumes:
Costumes for the most part are easily unlocked by two methods. One involves completing story mode with a character to get their next costume, while the other involves purchasing them at the Zack Shop under DOA 4 Live. Note: If this is your first time playing the game you should initially have 10,000 Zack points already, which is enough to purchase two costumes for whomever desired.