The following trick allows players to use the same character for upto three players in a four player deathmatch.
1. In 4-player multiplayer mode, goto character selection screen and set the character who is to be multiplied as 'Player 4'.
2. Choose characters for players 1, 2, and 3.
3. Back in the multiplayer mode screen, change number of players from 4 to 3. Now, go to the character select screen and set Player 4's character as Player 3. Pick any character for players 1 and 2.
4. Back in multiplayer mode screen, change number of players from 3 to 2. Now, go to the character selection screen and set Player 3's character as Player 2. Pick any character for Player 1.
5. Now, simply change the number of Players back to 4 and start the game. Players 2, 3 and 4 will be the same character.