Every lens has its purpose, and there's no "right lens" for you or anybody. It all depends on what you're looking for, and what you feel is lacking in your equipment.

What's "wrong" do you feel about what you have? Why do you feel you need the 70-200/4L? Is it because your 18-200 is too slow? Or not sharp enough? If you think it's too slow at F5.6, then why do you think one stop faster at F4 is good enough? Why not 2.8? And do you need IS?

The 70-200/4L is the perfect lens for somebody, but totally wrong lens for somebody else. No way can we know which one is you.

Since you mention Africa, then you probably want a lens that's longer than 200, which you already have.