what about the 24-120F4 ? At 120mm F4, should be reasonably good too ?
thanks
Let's not speak too much about bokeh, which is a subjective matter and is not always positively corresponding to aperture and focal length.
Isolation and DOF would be objective measures.
If you stand at a fixed spot and take out your 20mm, 50mm, 85mm, 200mm, without moving from the fixed spot, take a photo each of the same subject (say gf, wife, daughter) at the same setting (aperture fixed, maybe use a tripod to take away camera shake), then you crop the 20mm photo to match the 50mm, to match 85mm, to match 200mm - your resolution will drop like made, so don't worry about resolution for now - you will notice that the DOF and perspective are identical. DOF and perspective are related to the aperture and distance from you subject.
Ok, now fix your subject size and repeat the same exercise, this time due to the fixed subject size, you will be very near your subject with 20mm, further at 50mm, even further at 85mm and furthest at 200mm. Now with the same aperture you will get very much different DOF and perspective - you have changed your distance. And hopefully you are like our brother Light Machinery (I am guessing here but shouldn't be far off), you have 20mm f/1.8, 50mm f/1.8, 85mm f/1.8, 200mm f/2, and your subject is your lovely girlfriend (or wife or daughter depending on your stage of life - LM and I are shooting daughters now), you could shoot at f/2 and make that comparison.
You will find that, again not talking about bokeh, isolation is better with the longer focal length lens - this we all know. But if you stop down the lens to f/4, things are very very different here. 200mm at f/4 still has good to very good isolation, but 20mm at f/4 has enormous DOF. That's why we often see portraits taken at 200mm f/4 (like using 70-200mm f/4G VRIII) and get great results. As your focal length reduces, say to 135mm (or in your case 120mm), then isolation at f/4
may not be sufficient, but as internet speak goes, YMMV - your mileage may vary - depending on other factors, such as how beautiful your subject is (gf, wife, daughter?), what kind of lighting, how far your subject is from the background, what kind of background (busy, specular highlight?), and other factors I can't think about right now.
That's why for isolation as focal length reduces it is more and more important to have wider and wider aperture. So 200mm f/4 is sufficient for me (180mm f/2.8 is better!), at 135mm most prefer a f/2 lens, and at 85mm f/1.4 (for poorer man, f/1.8) is preferred, but at 50mm, f/1.2 is well liked (that is another story - spherical aberration gives a special look, and manual focus). And specially designed lenses, like the AFS 58mm f/1.4G, is the rage for those who have tried it and learned how to use it. I am speaking from experience, except I don't have AFS 58/1.4 and I don't have 200/2.
So the short answer to your question - it would be very much more challenging to use the 24-120 as a portrait lens at 120/4. Also speaking from experience, I bought the lens before Nikon lelong it with D750, had I waited I would have gotten it for half the price I paid earlier.