https://www.looper.com/1331931/teenage-mutant-ninja-turtles-4-what-we-could-have-seen-canceled-project/
The first "Teenage Mutant Ninja Turtles" film franchise nearly got a fourth film, only for things to fall through after the third film's poor reception.
I remember vaguely from rumors online, I think TMNT 4 had a weird storyline with the turtles mutations 'continuing', which was later developed into The Next Mutation TV Show (which they didn't have any new mutations).
However, the title of that show is a clue as to the concept for TMNT 4. I think you can find it on Eastman or Laird's blogs, concept art for the turtles in TMNT 4 movie. They were going to completely re-design the turtles. It wouldn't have gone down well with the fans - it would have been like the Power Ranger shows at the time - "Ninja Force" , "Giga Force" , "Dinosaur Force" and all that nonsense. As i understand, they were basically going that direction with the turtles. Give them a complete fresh look , new mutant powers, new outfits....
So for everyone hoping for a return to the form of TMNT 1990 which is usually considered the best TMNT film... this would have been as far from that as possible.
https://turtlepedia.fandom.com/wiki/TMNT:_The_Next_Mutation
Mike could morph into a human and Raph had raptor teeth and claws, etc...
Introducing a fifth turtle is a terrible idea, always was. It's not the freaking X-Men you can't just add new people.