I'd agree with #4. This was a tactic that the imperialists used almost everywhere, not just in Africa. They joined hands with a small group which was suddenly made to appear as having grown a lot more powerful as a result of its association with the imperialist. Slowly, it was used to overpower other groups around it as well as its own rulers, who were caught unawares.
If only people had come to realize that there are no free lunches, the imperialists would have had a much harder task to accomplish.
I think one of the key ideas to think about when discussing the rise of imperialism by the Europeans in Africa is the arbitrary creation of nation states that disregarded traditional ethnic boundaries. By ignoring tribal and ethnic ties and simply drawing the lines of countries on a map, the European nations created states that could not survive because they were constantly in conflict with themselves. Imperialist created countries in Africa found themselves not only at odds with the Europeans but with traditional enemies that they were now told were their fellow countrymen. Unfortunately the resistance to European exploitation (particularly economic) remained small and unorganized until the 20th century.
Of course, there are also numerous examples of the violent way in which indigenous African groups expressed their protest at the annexation and domination of Africa. The way in which Africa was viewed as a 'dark continent,' ripe for the filling, still remains a massively problematic issue today as impoverished Africa deals with nations that enriched themselves through Africa's resources.
One of the major ways that indigenous Africans reacted to imperialism was to try to use the imperialists to their own advantage. For example, various ethnic groups would compete to get the European nations on their side. If one of them could accomplish this, they could gain a great deal of power relative to the other ethnic groups.
Note: please refer to the rise of Imperialism in Africa starting in the 1500s and beyond.
Please, include any references also.