The problem is to convert the polar equation $r = \frac{-4}{\cos \theta}$ to rectangular coordinates.

1. Problem Description

The problem is to convert the polar equation $r = \frac{-4}{\cos \theta}$ to rectangular coordinates.

2. Solution Steps

First, we multiply both sides of the equation by $\cos \theta$:

$$r \cos \theta = -4$$

We know that $x = r \cos \theta$. Substituting this into the equation gives

$$x = -4$$

3. Final Answer

The rectangular equation is $x = -4$.