Owen Hughes’s review published on Letterboxd :
Billed as a sequel that follows on from where the last film ended. Sort of. Kinda. Some time has elapsed since the 3rd film, Casey and April live together, Leo has been sent off to the jungles of central America by Splinter to train on his own to become a better leader, Raph is doing his own thing, Mikey and Donny have jobs. The main story is about 13 monsters who, thousands of years ago, came through a portal to another dimension, opened by a warrior who blah blah; it's a bit of a non-starter to be honest with you. It's not reason enough to make you want to watch this. If you choose to watch it, it's because it's the TMNT, not because it has a lame plot about some immortal warrior bloke. In fact, I'd argue that the more you knew about it, the less likely you are to seek it out.
It's a pretty terrible film. The animation is ok, definitely has that cartoony style/vibe that it needed, but that's about as much praise as I'm willing to give it. Oh, alright, it also gets bonus points for Picard in it. I can see that they were trying to make it darker, but the level of audience they pitched the film at, it just doesn't work. I'd go so far as to say the first film has a darker tone to it. All this film does is suppress what makes the characters interesting in the first place, and serve up a really dull interpretation.
The turtles are just completely unrecognisable. They have only the very, very basic character traits that define them (leader, smart, goofy and grumpy) and that's about it. Where's the riffing off of each other? Where's the fun? Where's the originality? It's very much a kids film, with the minimal requirements it needed to make it worth seeing after watching the previous 3 in the series. It's just basically really naff and boring; I was very disappointed. I thought (/hoped) it would be a return to form after the previous instalment, but it wasn't.