The first review of the EF 400mm f/4 DO IS II has just appeared in ePhotozine
http://www.ephotozine.com/article/canon-ef-400mm-f-4-do-is-ii-usm-lens-review-26785
The sharpness at f/4 is phenomenal. The sharpness falls off with increasing f number, and rapidly above f/11. From the Canon MTFs, at 560mm with the 1.4xTC it is probably sharper than the 300mm f/2.8 II at 600mm with the 2xTC and slightly lighter. It is a superb lens - pity it is so expensive.
http://www.ephotozine.com/article/canon-ef-400mm-f-4-do-is-ii-usm-lens-review-26785
The sharpness at f/4 is phenomenal. The sharpness falls off with increasing f number, and rapidly above f/11. From the Canon MTFs, at 560mm with the 1.4xTC it is probably sharper than the 300mm f/2.8 II at 600mm with the 2xTC and slightly lighter. It is a superb lens - pity it is so expensive.