The new 400mm lens is very short so it must be a PF lens. The top looks about the same width as the 58 Noct, which suggests a 95mm filter size. If so, the maximum aperture would be f/4.5. With a 1.4x TC you would get a 560/6.3 lens. The lens/TC combination brackets the 500/5.6PF lens. I wonder why Nikon choose to make two telephotos which are so similar in focal length and speed?
In response to wondering about why Nikon makes a close range of telephotos, I think many lenses are in a ratio of square root of two (approx 1.4) - For example 25mm, 36mm, 50mm, 70mm, 100mm, 140mm, 200mm, 280mm, 400mm, 560mm, 800mm. Most people don't want to use a teleconverter if they don't need to.
Also, the new lens can be used on Z models without FTZ, also simplifying things. When this lineup is completed it is likely that the 500mm F mount will be near end of life.