Eh, unless every Dream Friend is carried over to the next game with some sort of significance outside of cameos, I don't think the "4th Main Character" thing will necessarily apply to the Kirby franchise, especially when you think about what makes Kirby, Meta Knight, Dedede, and (I guess subjectively) Bandana Dee main characters. Perhaps one could argue that Dream Friends like Marx and Gooey could be named as the "4th Main Character" within their own respective games, but I think it would be a little weird to deem a certain character as a "main" character if they just appear, disappear for a while (outside of cameos), and then show up again, especially when taking the entirety of the Kirby franchise, or at least each era into account.
It just opens up the floodgates for anyone to be called a main character just for merely existing, which clearly doesn't seem to be the case.
I just think it's much easier for a game like ARMS to get away with it, since it's the first entry in the series and no character particularly stands out from the others in terms of appearances or being more recognizable. Eventually though, they could easily ride off of the popularity of one character in particular before bringing them into a game like Smash.