The new Canon EF 200-400mm f/4L IS USM Extender 1.4x Lens is a telephoto-zoom lens for Canon EOS cameras that features a constant f/4.0 maximum aperture, Ultrasonic Motor focusing, three Image Stabilizer modes and an integrated 1.4x extender to effectively increase the maximum focal length to 560mm. The optical construction has been optimized and incorporates one fluorite lens and four Ultra-Low-Dispersion elements to minimize chromatic aberrations throughout the zoom range. Full-time manual focus override, AF stop buttons, a focus preset and a focus limiter function are available as well as a Power Focus mode to provide smooth, electronically controlled focusing that is well suited to video applications. The lens comes with a dedicated lens hood, accessory monopod mount, a drop-in gelatin filter holder and a fitted hard case for protecting and carrying the lens and its accessories. You can pre-order this lens from from B&H using this link for the price of $11,799.00.
Check this relatively one sided review of this lens by Andy Rouse who has spent 6 months with this EF 200-400 f/4L IS 1.4x at his blog here.