I know people don't like the Northampton arc but it's a good oppourtunity to give the Turtles some self-contained adventures and some breathing room from the whole plot. There's some really good Leo development, alongside an awesome 'vision quest' episode that I personally love. Season 3's got a lot of filler but the more dramatic episodes are top notch, especially the finale.
Season 4 has been a mixed bag so far. There's some great fanservice, such as the new Turtles meeting the 80's turtles, April gets the yellow jumpsuit, fucking BASED voice cast (they got David Tennant), but in it's current format it's leaving April feeling sort of unnecessary and Casey hair-tearingly irritating because he refuse to contribute to any plots in any meaningful way or even react to his situation realistically.
But yeah, keep watching. I love the show, and I'm glad it's getting attention on /co/