Now that the 20th anniversary of the original Ninja Turtles is coming along, I thought I'd cover...
Yes, there was a fifth Ninja Turtle. This came about in a Live-Action series by Haim Saban (Power Rangers) in 1997 called "Ninja Turtles: The Next Mutation." The show aired on FOX Kids and at the time Saban had property of it. The series supposedly followed the continuity of the live-action films. The Turtles lived in the same abandoned train station featured in the second and third films, and Splinter's ear was slashed as it was in the original film. But it did not include April O' Neal and Casey Jones and did include Shredder, who was dead in the films. I remember reading about her first in a Time Magazine article about females in the media such as Daria and Buffy.
The above image is an illustration from the official website back in 1997. She was part of an online dairy, it continued until 1999 but was then removed from the site. Her Story: Venus was one of the five turtles that were exposed to mutagen in the sewers. She wandered off and was discovered by a shinobi magician in China Town. Her name was Mei Pieh Chi, but when the main villains of the series--dragons killed her mentor, she returned to New York. In the struggle, the arms of a statue of a woman were broken off. After the turtles win the battle, the female turtle takes the statue with her to the lair. This earned her the nickname of Venus de Milo after the famous statue, mimicking the other Turtles' artist namesakes. In the series, Venus was not played by Nicole Parker of "MADtv," but an Australian actress called Nicole N. Parker.
Another unique thing about her and her 'New Mutation' brothers was that they met the Power Rangers, an occasion that was came about because Haim Saban was producing this Turtles incarnation and meeting the Power Rangers would be natural to promote the series. But the turtles was produced in Canada and because PR was done in LA at the time, they didn't used the suit actors and different voice actors for the Turtles. It was in Power Rangers in Space episode "Shell Shocked" in 1998. But it was futile to even promote on Power Rangers because the original fans of PR had either left by season 3 (1995-1996) or Turbo (1997). So not many fans of the 1989 TMNT animated series were not aware of this teamup.
Now, the controversy came in with the Ninja Turtles creators Kevin Eastman and Peter Laird. In the Next Mutation, it was explicitly made out that the turtles were not directly related and of course not biologically related to Venus. The writers could give her a romantic relationship between Venus with the rebellious & quick-tempered Raphael and/or the stalwart & fuddy-duddy leader Leonardo. This made Laird mad because he stressed that the turtles were the only and last of their kind, and if there was a chance of hope at procreation through a female turtle, that would defeat the fact. Director Kevin Munroe elaborated in an interview on the instructions Peter Laird gave to him for the recent CGI TMNT. Munroe admitted that among those rules was, "There’s absolutely no mention of Venus de Milo, the female Turtle. You can’t even joke about that with Peter. It’s just one of those things that he hates with a passion." Laird has said, "As long as I am in charge of this ship of Turtles, Venus de Milo will never be mentioned again." 
It is said that FOX Kids wanted to add the female turtle and Laird was against it, but had no choice. Contrary to popular belief, the show Next Mutation actually had nice ratings. FOX Kids simply canceled the show, I think there was a miscommunication or problems with production. I have always liked female incarnations in an established male franchise and she is an interesting lark, even though we may never see any new official form of her.
