I really have come to consider the 400 f4 PF should be one of Nikon's top priorities..... And then why not follow up with 600 f5.6 and 800 f8 PF in Z-mount [ie 100mm window]?!?
In phase-fresnel R&D we can be certain about two data points in lens allometry (weights, outer dimensions, filters etc) : 300 f4E PF = 755g, 148 x 89mm; 77mm filter on 75mm window, and 500 f5.6E PF = 1460g 237 x 106mm, 95mm filter on 89.3mm window. So it follows a 400 f4E FL needs a 100mm window, and 105mm filter thread [a rear 40.5 filter slot makes far better sense as in the standard super-telephotos].... Nikon's 400 f5.6 PF patent lists 22mm OL.
Let's face it, if they did a 400 f5.6 it's hard to make it shorter nor lighter than the 300 PF + TC14 [ie 420 f5.6 [a great little combo with superb IQ, as is a 510 f6.7 with TC17 ]. In contrast, a 400 f4 has a decent Teleconverter Factor -
the TCF - going out to 800 f8, which should AF rather decently on a MILC / hybrid DSLR. I say this judging from the 500 PF + TC14 III on a Z7 or even D850; and I have pushed the 500 PF with TC17 on a Z7 - and obviously on D780 in L'view. In any case, with TC14 on a 400 f4 the effective reach of 560 f5.6 edges close to a 600 f5.6. Albeit, the latter will probably beat it on IQ; equally, extending 600 f5.6 PF+TC14 to a 840 f8 will also have better IQ than 400 f4+TC2 III.
These well known IQ penalties with the F-Nikon system could well be eclipsed by the new pair of Z Teleconverters. So to conclude on choosing between 400 f4 or 600 f4 (?) Let's consider several factors - including (1) the Z winds of change, (2) manufacturing constraints in PF technology, (3) allometry aka scaling up lens haptics, and (4) above all the market(s) for nifty telephoto lenses. And (5) don't let's forget the swelling demands for airline-friendlier gear that's easier on skeletons and joints...
Above all, the photographer is that much likely to grab the Proverbial Moment if out and about with a Nikkor PF. These have a tendency to be the telephoto one has in one's hand.
Together, they say 400 f4 PF. Dammit, flocks of Birders and herds of Peripatetic wildlifers will stampede to own a 400 f4 PF S Nikkor in Z-mount. [EDITED February 2023, Nikon released the dinky 400 f4E.5 S not PF but a success on key factors ]
Such a prime remains a mere hypothetical fantasy until Nikon releases a credible MILC for action. Seriously, it is nothing less than remarkable how Nikon have failed to consolidate their unique advantage with the 300 PF and 500 PF. Birding is a huge business as are safaris (viruses permitting that is). All this goes to show misleading distortions in their marketing data: ie when will Tokyo stumble to the facts that longer telephotos ain't solely for sports.