I want to trade my Canon 400 5.6 L lens on this. (Canon EF 300mm f4.0L IS USM Autofocus Lens USA) I will gain image stabilization and a fixed f4. I will be able to use my Canon 1.4 extender and still achieve auto focus. I do wildlife photography. I also have a Canon 100-400 5.6 L. lens. Your thoughts on my trade?
Can't speak to the 400/300 comparison. Can tell you that I'm *very* happy with the (remarkably light) 300 for the reasons you mention. Works great with the 1.4 extender, with just a little sharpness lost at the edges...though for wildlife, the edges often get cropped anyway.
given the IS with f4, hummingbirds are easy. I use the 300 a lot. If you already have the 100-400, seems like an easy call to me. Hard to see what you lose, easy to see what you gain.