4. (10 points) The cheetah runs a step length of 2 meters, the fox runs a step length of 1 meter. In the time it takes the cheetah to run 2 steps, the fox runs 3 steps. The cheetah is 30 meters away from the fox, then the cheetah needs to run ( ) meters to catch up with the fox.
A. 90
B. 105
C. 120
D. 135
【Analysis】In the time it takes for a cheetah to run 2 steps, a fox can run 3 steps, meaning in the time the cheetah runs $2 \times 2=4$ meters, the fox runs $1 \times 3=3$ meters. Since the time is constant, the speed ratio is the inverse of the time ratio. Therefore, if we set this time to 1 second, the cheetah's speed is 4 meters/second, and the fox's speed is 3 meters/second. Then, by dividing the catch-up distance of 30 meters by the speed difference, we get the catch-up time, and then multiplying by the cheetah's speed of 4 meters/second gives the required distance.

【Solution】Let the cheetah's speed be: $2 \times 2=4$ (meters/second), the fox's speed be: $1 \times 3=3$ (meters/second),
$$
\begin{array}{l}
30 \div(4-3) \\
=30 \div 1 \\
=30 \text { (seconds) }
\end{array}
$$
$$
4 \times 30=120 \text { (meters) }
$$

Answer: The cheetah will run 120 meters to catch up with the fox.
Therefore, the correct choice is: $C$.