As long as you don't need any depth of field, like taking a picture of a mountain range on the horizon, shooting at f/4 probably would work well enough. However, that is not my normal style when using a long lens for landscapes.
However, no procedure without a break to it, thus I'll give the f/4 a spin. Over the weekend I might be able to test-run the 200-400 at wide open for more regular landscapes, though, if our weather deities are condescending enough.
Still, I consider shooting at f/11 and mounting the lens on a really good tripod is the better approach.
Bjorn -
Thanks - I would indeed appreciate any wide open distant images - f/11 is not for me as most of my landscape shots are at dusk or dawn and there is usually a moving element in the image - ship, ferry, waves, clouds, etc. and that's why I want f/4 capability - to get the shutter speed up and/or ISO down when image movement is a factor.
Also, when shooting lights at night and wanting sunstars, one usually needs to stop down one from wide open and if wide open is f/5.6 that can be a problem sometimes when f/8 is too slow or too high an ISO for what's going on.
Most of my subjects are many miles away so 400mm DOF at f/4 is just fine most of the time.
EDIT: One could say the 400/3.5 is for me, but I've not been impressed with that lens when I tried one copy out as Bjorn previously alluded to.
400/5.6 ED-AI at f/8, 4 sec, ISO 100