Alastair Oswald is investigating "A sense of place: sensory perceptions of Neolithic causewayed enclosures in their landscape contexts"and notes : "The observation that many causewayed enclosures ‘tilt’ across the contours, with the result that their viewsheds are restricted by higher ground, was first made by Isobel Smith more than forty years ago (Smith 1971, 92). Smith interpreted this phenomenon as evidence that each monument was designed to be intervisible with a specific lower-lying area, perhaps equating to a 'territory' exploited by its builders. Despite a subsequent increase in the number of known upland sites, the observation still holds good for many, so Smith’s inference has been amplified (Oswald et al. 2001, 91-102) and is now accepted by key authorities (Healy 2004, 31; Mercer 2009, 766; Whittle et al 2011, 12)."
I have Anquet's OMN on my system which allows me to plot route elevations, which give sightlines and it seemed that at the very flat summit of Milk Hill there was a possibility to find a place where all four were visible, though Robin Hoods Ball being 18 km away the visibility of it may be considered more theoretical to the naked eye.
There is, it is a very small area, there is permissive public footpath across the field to the summit so I urge you to find it for yourself, and with the right camera a much better photograph than my panorama may be possible.
(2019 Update) - Since this was first published another causewayed enclosure has been discovered at Larkhill and checking the elevations it appears that it too would have been visible from the same spot. Five causewayed enclosures visible from one place.
Click to enlarge