A 400mm f/2.8 lens, or a 400/5.6? That'll make a difference. For the latter, even the lightweight and (relatively) inexpensive Bogen/Manfrotto 190CX or 190CX Pro will do. For the really big lenses, the beefer Gitzo and Induro tripods are better suited. I have a Gitzo 3530LS for my heavy stuff, and the aforementioned 190CX for small stuff. Both work well.