Recently found this nice webpage where You can calculate the background blur of your Telelens-Camerasensor combination:
Very handy to get some ideas on how the isolation power of the various lenses behave in comparison. If You look carefully the 400mm f2.8 and the 600mm f4 seem to create the most blur but its distance and background-distance depended. It also doesn’t tell you anything about the blur QUALITY and/or depth of field in that context. If you however always choose the same size of your object/subject in your frame (modify the distance to the object with different lenses) and you keep the same aperture – the resulting DOF should be basically the same.
Anyways – the 400mm f2.8 looks really as if it is the blur king in these medium distances!