Because Kobayashi hasn't worked for Square Enix for ten years and suddenly said she submitted new artwork for them? This could very much mean a new SaGa game since she is not associated with anything else other than the SaGa series.
That and like others said, the '4' and the background GB-like music might be related a 'SaGa 4'.
Of course, she could always be doing the artwork for something else, as already brought up in this thread, but the point of the thread is to discuss the possibilities, I'd think.