February? Intersting time. The grass should be quite high and green, meaning plenty of prey, meaning plenty of predators!
I'm on my 4th next September, and having upgraded for each in terms of cameras and lenses, I would thoroughly recommend the 100-400, as the extra 100mm+ is invaluable. It has quick enough focussing and plenty of reach. I appreciate this is heavier than the 70-200f4, but so worth it. If you start adding extenders to the 70-200, you don't even get 300mm and you start wandering into the realms of lower IQ and smaller apertures, longer exposures and "that killer shot" being blurred, as animals tend not to respond to requests for recomposing a shot, and I've not yet met a hyena that responds to "sit" (I have tried).
My advice would be:
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[*]Zoom not prime, as it gives you much greater flexibility.
[*]BUY a used 100-400, use it and then sell it - cheaper than renting, and we made a profit both times, and even if you lose money it will be less than renting and insuring the rented lens.
[*]Get a shoulder stap, e.g. Sun Sniper as that will take the weight off your neck
[*]Get a cheap second body and stick a wide-angle zoom (24-70 or 17-85, even the 18-55 if you've not got a lot of cash) on it as aminals have a habit of sneaking up on you when you've got the wrong lens, and if you are going to Kenya you're likely to get within a few feet, also great for sunsets.
[*]Spend time looking with your eyes, not your camera.
[*]Don't where black or blue if you're going to a tsetse fly area.
[*]ENJOY!
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At the end of the day, you can never have a long enough lens. I was chatting to a guy last year who was running a 600mm on a 7D. As far as he was concened that was too short (effective 600x1.6 crop = 960mm)!
Grant