We've heard, along with others that the EF 400 f/5.6L is hard to acquire in a few areas around the globe. This is leading to the suggestion that the little supertelephoto is being primed for replacement with an IS version.
We have said we've been told 2014 will be the “year of the lens“, and if that's the case, a few of the lenses will definitely be of the niche variety. The lenses have lower production requirements and usually higher margins. The EF 400 f/5.6L, while a wonderful performer, would certainly benefit from the latest optics, IS and perhaps a bit of weight savings or length reduction.
Canon EF 400 f/5.6L at B&H Photo