I LOVE the 'Ethiopian' musketman!

It's perfect, although I think it fits Kanem-Bornu at least as well as Ethiopia.
As for the 4 most important civs, I think you can group them a little to get...
For sure:
ETHIOPIA, it was both a great power and a center of culture
MALI, given that it was preceded by
Ghana and followed by
Songhai in the same region and with similar traditions in the western Sudan
Probably:
NUBIA, with its immensely long history and culture (and a great rival for Ethiopia - I'd be tempted to group them with Ethiopia in a way, but they looked toward Egypt and Ethiopia looked toward Arabia in cultural and political matters so they actually make a great contrast)
and then your pick of:
KANEM-BORNU, which was a regional power and one of the first states to adopt Islam. It had a continuous political history of close to a thousand years and around 1600 was the greatest power in Africa (which makes this musketman a beautiful unit for them

) Not well known to the west because of its location dead center in Africa but traded and exchanged embassies with the Ottoman Turks. OR...
The
SWAHILI cities, a rich trading community with an Islamic culture but predominantly African in population and character (and a great SEAFARING candidate in CIV3 by the way)
I also considered
KONGO* for its fascinating history (adoption of Christianity and western culture) and several western African civs, but settled on those above due to their duration and political, economic, and cultural influence.
*P.S. '
Kongo' in this instance is most closely associated with the area and peoples of modern
Angola