$(GBR 4)$ The segment $AB$ perpendicularly bisects $CD$ at $X$. Show that, subject to restrictions, there is a right circular cone whose axis passes through $X$ and on whose surface lie the points $A,B,C,D.$ What are the restrictions?
1. **Given Setup**: We are given that the segment \( AB \) perpendicularly bisects \( CD \) at \( X \). This means \( X \) is the midpoint of \( CD \) and \( AB \perp CD \).

2. **Midpoint and Cross Ratio**: Let \( M \) be the midpoint of \( AB \). If \( X \) is also the midpoint of \( AB \), then \( CD = AB \). Otherwise, let \( Y \in AB \) be such that the cross ratio \((X, Y, A, B) = -1\) is harmonic. 

3. **Circle on Diameter \( XY \)**: Let \((P)\) be the circle on diameter \( XY \). For any point \( V \in (P) \), \((P)\) is the V-Apollonius circle of \(\triangle VAB\). 

4. **Incircle and Excircle**: The incircle \((I)\) of \(\triangle VAB\) touches \( AB \) at \( E \) and the V-excircle \((J)\) touches \( AB \) at \( F \). The tangents of the Dandelin spheres \((I)\) and \((J)\) from \( V \) are elements of a circular cone with axis through \( X \).

5. **Ellipse Intersection**: The plane \((AB, CD)\) cuts the cone in an ellipse with major axis \( AB \), center \( M \), and foci \( E \) and \( F \). 

6. **Movement of \( E \)**: Without loss of generality, assume \( AX > \frac{1}{2}AB \). As \( E \) moves on \((P)\) from \( X \) to \( Y \), \( E \) moves on \( AB \) from \( X \) to \( B \).

7. **Length of \( CD \)**: Consequently, the length of \( CD \) must be less than the latus rectum of an ellipse with major axis \( AB \), center \( M \), and one focus \( X \). The latus rectum of an ellipse with major axis \( p \) and minor axis \( q \) is given by \(\frac{q^2}{p}\).

8. **Calculation of Latus Rectum**: For the ellipse with major axis \( AB \) and one focus \( X \), the length of the latus rectum is:
   \[
   \text{Latus rectum} = \frac{AB^2 - 4MX^2}{AB}
   \]
   where \( MX \) is the distance from \( M \) to \( X \).

9. **Restriction on \( CD \)**: Therefore, the restriction on \( CD \) is:
   \[
   0 < CD < \frac{AB^2 - 4MX^2}{AB}
   \]

The final answer is \( \boxed{ 0 < CD < \frac{AB^2 - 4MX^2}{AB} } \).