The scale given is 4 inches = 3 feet. To express this as a scale factor, we need to convert both quantities to the same unit.
First, we'll convert 3 feet to inches. Since there are 12 inches in a foot:
\[ 3 \text{ feet} = 3 \times 12 \text{ inches} = 36 \text{ inches} \]
Now we have the scale in inches:
\[ 4 \text{ inches} = 36 \text{ inches} \]
To find the scale factor, we set up the ratio of the drawing measurement to the actual measurement:
\[ \text{Scale factor} = \frac{\text{Drawing}}{\text{Actual}} = \frac{4 \text{ inches}}{36 \text{ inches}} \]
Now, we can simplify this fraction:
\[ \frac{4}{36} = \frac{1}{9} \quad (\text{dividing both numerator and denominator by 4}) \]
Thus, the drawing's scale factor is:
\[ \boxed{\frac{1}{9}} \]