For wildlife photography, this lens will give you the most bang for your buck. I shoot birds in flight with this lens handheld. No problems. Some people say that for a 400mm lens without IS that you need to use a tripod. I would have to strongly disagree. With good lens holding technique and practice, this lens is easily handholdable. Since it is so sharp wide open, this allows for fast shutter speeds. The only time I stop down is for more depth of field when I am shooting insects or small birds.
Add a 36mm kenko autofocus extension tube for semi macro photography. I shoot larged sized insects (wasps, butterflies, dragonflies, praying mantis) or small birds with amazing sharpness and depth of field. This combo makes for a great backyard lens. I also go hiking with it at the local wildlife sanctuary without any problems. If you enjoy birding photography, this is THE lens to get.
Yes you can get the 500mm f4 but you are going to need a sturdy tripod and you are gonna need a really fat wallet because that lens is crazy expensive.