4. As shown in Figure 8, point $A$ is a grid point (vertex of a small square) in a $5 \times 5$ grid. Each small square in the grid has a side length of 1. How many grid-point isosceles right triangles (with all three vertices being grid points) have an area of $\frac{5}{2}$ and one vertex at point $A$?
(A) 10
(B) 12
(C) 14
(D) 16
(提示: The area of an isosceles right triangle is $\frac{5}{2}$, then the two legs are $\sqrt{5}$.
$$
\text { Point } A \text { is }
$$

the right-angle vertex
(as shown in Figure 9
(a), or
point $A$ is the base
vertex (as shown in
Figure 9 (b)), there are 8 right triangles in each case,
a total of 16.)