If you just received a code for the PlayStation 3 beta of PlayStation Now but would rather try the PS4 version, you may want to try redeeming it directly on your PS4, because it will actually work, as reported on Reddit.
A few screenshots of the new PS3 lineup have also been leaked.
Update 2: apparently the information provided earlier by Sony's reps is incorrect, as multiple users are reporting further conversation with other reps saying that PS4 beta code hasn't been sent out yet, and will be in two weeks. Everyone that received a PS3 beta code and also marked "PS4" during registration will receive a PS4 code within that time frame. If you registered your code on PS4 you can still use it on PS3, as it'll still be valid.