Nothing is leaving for Strange New Worlds at this moment, there are five Star Trek series that will air in 2022, and all of them are getting at least another season after that.
Now, there are 5 series currently, and no one has ever said there would/could only be 4 series. That’s never been a rule, or an idea, presented by Kurtzman.
He did say that the next show in development to move to series would probably wait until one of the current series has ended, I would think because he’s okay with 50-60 weeks of Trek a year/production cycle, as that keeps Star Trek on all year round, and an additional series, even with a decreased episode order, require a lot more resources to pay all the members of creative and production, and actors and fabricate new sets, props, and costume assets.
Are they beholden to a four series rule? No, that’s never been a thing. Are they sticking to just five series for now? Yup, but they’re under no obligation to continue with only five for any length of time, and could easily revise their plan.