if you do a search, you will find lots of comments on this subject, including some of mine
if you're definition of wildlife includes birds and small animals, then 400mm on a crop-frame - 640 eff is often not long enough. however alomost anything longer is too heavy and too expensive or and of poor resolution (Sigma lenses and the Tamron 200-500 which i sold) and requires a sturdy tripod
the Canon 300 f4 is not long enough and mine doesn't perform well with a 1.4 extender (unless you have a camera with micro-adjust, you're very unlikely to get decent autofocus performance from an extender - the tolerance build up of extra glass and mechanical interfaces are beyond Canon's production capability - and i can't get get good resolution even with micro-adjust)
the Canon 400 f5.6 is super sharp corner-corner but without IS it can be hand - held only at 1/1000 sec or faster - broad daylight only, not the best time or light for a lot of wildlife. this is mostly a tripod lens which will be too long for larger/closer animals - even birds in flight
by process of elimination it's the 100-400, warts and all. it's versatile, hand holdable with IS, and makes consistently good 13x19 prints (at f8 with a poor copy, wide open at f5.6 with a good copy). i've used the 100-400 in Africa, Australia, Borneo, Brazil, and many other places and gotten images that would not have been possible with a long prime or non-IS lens.
