A great pub quiz question. Via.
If you have time, try taking the You Don't Know Africa quizzes.
Addendum: Apparently they don't exactly meet. An unknown reader provided a link to Kazungula -
At Kazungula, the territories of four countries (Zambia, Botswana, Zimbabwe, and Namibia) come close to meeting at a quadripoint. It has now been agreed that the international boundaries contain two tripoints joined by a short line roughly 150 metres (490 ft) long forming a boundary between Zambia and Botswana, now crossed by the Kazungula Bridge. The ever-shifting river channels and the lack of any known agreements addressing the issue before 2000 led to some uncertainty in the past as to whether or not a quadripoint legally existed...
More on worldwide quadripoints.