To find the base length of a triangle when you know the area and the height, you can use the formula for the area of a triangle:
\[ \text{Area} = \frac{1}{2} \times \text{base} \times \text{height} \]
In your case, the area is 10 square feet and the height is 4 feet. We can set up the equation as follows:
\[ 10 = \frac{1}{2} \times \text{base} \times 4 \]
Now, solving for the base:
\[ 10 = 2 \times \text{base} \] \[ \text{base} = \frac{10}{2} = 5 \text{ ft} \]
Therefore, the base length is 5 ft.