Enter the following at the password screen:
There is another way to unblock these four characters. Beating the original TMNT Arcade Game also we will obtain to Casey Jones, Karai, Splinter and Slashuur.
When on the rooftops in stage 9-1, start smashing the many crates to find an artifact. Take that artifact back home, and give it to April. Once she appraises it, she'll tell you that it is the original TMNT aracde game. Now go to the title screen, and go under options. The original TMNT arcade game will be there.