It is hard to move further up, the size and weight of 400mm f/2.8, 500mm f/4, 600mm f/4 are just beyond my way of photography. Can Nikon just make a good 400mm/4 as Canon has done? Nikon's answer for that is the 200-400mm f/4, weight 7.4 lbs. I almost made the decision of purchasing the lens for our upcoming African trip before realizing that there is another option. An option that probably everybody knows except me.
The option is AF-S NIKKOR 300mm f/2.8G ED VR II with teleconverter TC-14E II and TC-20E III. Nikon introduced the lens and teleconverters together several years ago and since further upgraded them. The lens weights 6.4 lbs and is a legendary lens from Nikon, one of its best. TC-14E changes the lens to 420mm f/4 with hardly any noticeable deterioration in image quality and focusing speed. TC-20E III changes the lens to 600mm f/5.6, and the performance is still good.
200-400mm f/4 is good with TC-14E II, but has focusing issue with TC-20E III, although with newer camera such as D4, it may work. But this issue tipped the balance and I opted to go for the AF-S NIKKOR 300mm f/2.8G ED VR II. It is still a big lens but with VRII, it is completely possible to shoot offhand. With the lens hood on, TC-14E II and D4 attached, it fits comfortably in my Think Tank Streetwalker HarDrive backpack. I could walk around with the set up for hours without fatigue.
200-400mm still has its advantage as a zoom lens, it would be hard to imagine changing teleconverters during the rain or snow. But that is something that I have to learn to live with, the curse of prime lens!
NIKKOR 300mm f/2.8G ED VR II, TC-14E II and D4: f/5.6, 1/500, ISO280