*Ni, (25 minutes) In space, there are $n$ planes $(n \geqslant 4)$, no two of which are parallel, and no three of which are concurrent. Among their pairwise intersection lines, what is the maximum number of pairs of skew lines?
Let $1,2, \cdots, n$ represent $n$ planes, and let $(i, j)$ be the line of intersection between plane $i$ and plane $j$. Let $a_{n}$ be the maximum number of skew lines among the pairwise intersections of $n$ planes. When the $(n+1)$-th plane is added, the total number of intersection lines increases by $n$ lines: $(n+1,1),(n+1,2), \cdots,(n+1, n)$. These $n$ lines have no skew lines among them (all are within the $(n+1)$-th plane). Note that the intersection line $(n+1,1)$ is not skew with the original $n$ planes' intersection lines $(1,2)(1,3)$, $\cdots,(1, n)$ (all are in plane 1), so $(n+1,1)$ can only form skew lines with the intersection lines of planes $2,3, \cdots, n$, and thus $(n+1,1)$ can form at most $C_{n-1}^{2}$ pairs of skew lines with the original intersection lines.

Similarly, $(n+1,2), \cdots,(n+1, n)$ can each form at most $C_{n-1}^{2}$ pairs of skew lines with the original intersection lines. Therefore, after adding the $(n+1)$-th plane, the maximum number of skew lines among the intersection lines increases by $n \cdot C_{n-1}^{2}$ pairs.

Since $n \cdot C_{n-1}^{2}=n \cdot \frac{(n-1)(n-2)}{1 \cdot 2 \cdot 3} \cdot 3=3 C_{n}^{3}$, we have the following recurrence relation:
$a_{n+1}=a_{n}+3 C_{n}^{3}$, and it is clear that $a_{1}=a_{2}=a_{3}=0$. Substituting 3,4, $\cdots, n$ into the above formula, we have
\[
\left.\begin{array}{l}
a_{4} \leqslant a_{3}+3 C_{3}^{3} \\
a_{5} \leqslant a_{4}+3 C_{4}^{3} \\
\cdots \cdots \\
a_{n} \leqslant a_{n-1}+3 C_{n-1}^{3}
\end{array}\right\} \stackrel{\text { n1mm }}{\Rightarrow}
\]
\[
\begin{aligned}
a_{n} & \leqslant 3\left[C_{3}^{3}+C_{4}^{3}+\cdots+C_{n-1}^{3}\right] \\
& =3\left[C_{5}^{4}+C_{5}^{3}+\cdots+C_{n-1}^{3}\right]=3 \cdot C_{n}^{4} .
\end{aligned}
\]

Therefore, among the intersection lines of $n$ planes, there are at most $3 C_{n}^{4}$ pairs of skew lines.