What African nation has its capital located at 6 degrees North and 4 degrees East?

The closest capital to that location is now an ex-capital : Lagos, in Nigeria.

It is near 6.45° N, 3.40° W.

There's no capital, city, or nation at exactly 6° N latitude, 4° E longitude. That point is at sea in the Bight of Benin, about 29 miles due south of the southern coast of Nigeria.

Nigeria's new capital, Abuja, is well inland, about 310 miles northeast of that point.