I went to the 'Focus on Imaging' exhibition at the NEC Birmingham (UK) today, while there I waited to get access to the elevated Canon stand to view their larger lens. Once up there, I checked the available lens to try on the stand, and I asked the Canon rep if there was the Canon 200-400f/4L IS to look at, he disappeared and came out with the Canon 200-400 f/4L attached to a Canon 5DMK3 which looked lost on it
No photos could be taken on a media card, but could shoot away focusing with it and changing the focus distance between the 1 and 1.4 switch.
I was impressed, even got the other half to photo me
I then asked the damaging question, how much, he replied it didn't exist. About the same time another college asked him to remove it from the stand, it seemed it shouldn't have come out, so I was lucky. Well, it was certainly nice trying it and it was not as heavy as I thought it would be hand held.