4. If each face of a tetrahedron is not an equilateral triangle, then the minimum number of edges with unequal lengths is
(A) 3
(B) 4
(C) 5
(D) 6
4. 【Analysis and Solution】Since each face of the tetrahedron is not an isosceles triangle, there must be at least three edges with different lengths. On the other hand, a tetrahedron with each pair of opposite edges equal can be a tetrahedron with all edges of different lengths and each face not being an isosceles triangle.