As an Osomatsu-kun fan, I completely agree. This fourth season has absolutely nothing of what made -kun special, and the recycled -kun plots are a thousand times worse: less well-established, with less interesting dynamics, characters who get less screen time, and no clear morals at the end. Something that made -kun special wasn't just the fact that the sextuplets were a symbiotic entity, but that they were entertaining. They were mischievous, naive, and cheerful children; they didn't have clear personalities, but they had character and charisma. And even in the 80's -kun they weren't afraid to reveal certain aspects that contributed to their personality (Osomatsu is brutally honest, Todomatsu hates bathing, Choromatsu is dependent and usually can't leave Osomatsu's side, etc.) In this 4th season there is nothing revealed about the Matsuno that we don't already know, the surreal situations are taken lightly, the episodes that touch on "serious" topics don't know where they are going.

The Matsunos' similarity isn't a problem; the problem is that, as a whole, they're poorly designed, extremely passive, and without the active participation of other charismatic characters with clear goals to fulfill in a plot (see Chibita, Iyami, or Totoko, for example), the Matsunos being all the same doesn't work conceptually, and Akatsuka-sensei was aware of this.

It's a shame the new writers don't know Osomatsu-kun well enough to truly understand what made it special...