To find out how far Keith drives using 4 gallons of gas, you can use the function given:
\[ D(g) = 25g \]
Now, substitute \( g = 4 \) into the function:
\[ D(4) = 25 \times 4 \]
Calculating this gives:
\[ D(4) = 100 \]
So, if Keith uses 4 gallons of gas, he drives 100 miles.