Kind of makes the thread pointless. Canon does offer a 400mm f/4 IS.
No red ring, but I see this at the local NBA games all the time so it must be ok.
It was a common discussion point before the MkII supertele lenses came out, when people felt the 'DO premium' means a non-DO 400/4 would be substantially cheaper. At this point, a 400/4 non-DO would have the built/optical qualities of the MkII lenses, and cost more than the current DO.
Worth noting that Canon has patented DO teleconverters, and IIRC several new DO supertele designs.