To find the rate at which Sam walked, we can use the formula \( r = \frac{d}{t} \), where \( r \) is the rate, \( d \) is the distance, and \( t \) is the time.
Given:
- Distance \( d = 4 \) kilometers
- Time \( t = 50 \) minutes
Now, substitute the values into the formula:
\[ r = \frac{4 \text{ km}}{50 \text{ min}} = 0.08 \text{ km/min} \]
Therefore, the rate at which Sam walked is 0.08 kilometers per minute.