On the whole I really enjoyed Season 4, but I agree with most people that it's not what it used to be.
Firstly, we're intoduced to new Misfits; Finn, Jess, Alex and Abby. For the most part they follow the convention of all being on community service, all with powers (Alex being the exception, well for now at least). The problem is, this all seems inevitable; misfits HAVE to be on community service right? They all need powers right? I think the writers missed an opportunity to play around with the format even more, rather than just trying to replace the missing cast. What if nobody was on community service? What if the beloved community centre wasn't featured any more?
I liken it to the James Bond series; does he always need to order martini's "shaken, not stirred", gadgets and girls with punny names, or is the character more than that. Misfits is more than orange jumpsuits, and this could have been a time to explore that.
Back to the newbies; Finn and Jess both have the obligatory powers (telekinesis and x-ray vision respectively), while I like the fact that the powers, like the original gang, seem to stem from their personalities, there's a key difference: they can control them. While Finn can't necessarily throw furniture across the room (yet), he consciously activates his power, as does Jess; this means that it's a lot harder for their powers to get them into trouble, or act as a catalyst for the plot.
Think about Kelly hearing her fiance's thinking she was a slut and dumping him, Alisha being forced to grow up because she couldn't use sex to get her way anymore. None of these plots or character development could have happened if they chose when to use their powers.
Another trend which began to emerge in Season 3 was a far more episodic type of storytelling. While Seasons 1 & 2 had specific plots and characters for each episode, it also larger arcs (Would the authorities or Sally discover they'd killed Tony? What was the nature of Superhoodie?) When episodes can be watched in isolation, in a different order, it loses momentum and reality. An arc gives the characters purpose, shapes the season and can build to a satisfying climax.
While I enjoy seeing different powers and characters, when they disappear in nthe same episode, the threat and therefore the drama is lessened. At times, this can be jarring: Fin meets a step-sister in Episode 5 who we never see again, despite the bonding that took place. Seeing Nathan's Mum and Dad several times, helped explain Nathan as well as made him feel like a believable. Whereas Finn had a girlfriend who he kept tied up, a sister we never saw again, a dead real Father, an ambivalent adoptive Father and a sex-crazed step-mum. None of this rang true to me; and the less believable the character seems, the less you care.