4. In a regular tetrahedron $A B C D$, $M, N$ are the midpoints of $A B, C D$ respectively, then the angle formed by $M N$ and $A D$ is
A. $30^{\circ}$
B. $45^{\circ}$
C. $60^{\circ}$
D. $90^{\circ}$
4. $\mathrm{B}$ places the tetrahedron $A B C D$ into a cube with edge length $\frac{\sqrt{2}}{2}$, such that each pair of opposite edges of the tetrahedron becomes a pair of face diagonals of the opposite faces of the cube $A_{1} C B_{1} D-A C_{1} B D_{1}$.
Then the angle between $M N$ and $A D$ is the same as the angle between $A E$ and $A D$, which is $45^{\circ}$.