For the bazillionth time, a 4 cylinder engine would cost too much. It's no less expensive to build a 4 cylinder 250cc than it is to build a 4 cylinder 1000cc. The parts count is the same. Sure, the 250 engine will be lighter so the materials cost is theoretically less - but the cost of the materials is trivial compared to the cost of engineering, tooling, machining, and assembly, and those are more a function of the parts count.
People complained that the CBR250/CBR300 was just a single cylinder compared to the Kawasaki (and now Yamaha) twin ... Honda had to respond. So a twin it is.
And we don't know whether this will replace the CBR300 in the lineup. I have a funny feeling that the naked/adv versions of the single-cylinder 300 will stay as is but the CBR version will go away and be replaced with this. And it could very well end up with a displacement bump for the North American market given that Kawasaki and Yamaha have both done that.