In addition to his comments, there is information on those 3 superwide alternatives in the Hasselblad system on the zeiss website
www.zeiss.de/photo in the Hasselblad series 500 section and in the Camera Lens News (CLN)archives.
In addition to this, I am preparing a web representation of my test photos from the Zeiss prototype testing a few years ago, which compare distortion, close-up sharpness wide open and stopped down, as well as vignetting.
Within Zeiss, I was the driving head behind coming up with an improved 40 mm lens for the SLR viewfinder (even though I like the Biogon 38 very much). Compared to the previous 40 FLE, I wanted significantly more sharpness at infinity for landscape and "citiyscape" shooters, sharpness on the level of the 50 FLE - at least. I wanted to get rid of the FLE mechanics with its confusing two focus rings. I wanted a single one. And I wanted it to operate smoothly, similar to the great focusing mechanism in the Tele-Superachromat 350, which I consinder the best focusing mechanism anywhere in the photo industry. On top of that I wanted a good offering for professionals who use digital backs and enjoy the Makro-Planar 120 for their commercial photography, but needed something significantly shorter in focal length. All of this has materialised in the new Distagon 4/40 IF - the first super wide angle lens for the SLR which can sucessfully challenge the Biogon 38.
If you have the opportunity to use it with your best tripod on landscapes with very fine detail at a far horizon, the result will stunn you.