To find the area of land needed to plant 20,000 seeds at a density of 4,000 seeds per acre, you can use the following formula:
\[ \text{Area (acres)} = \frac{\text{Total seeds}}{\text{Seeds per acre}} \]
Plugging in the values:
\[ \text{Area (acres)} = \frac{20,000 \text{ seeds}}{4,000 \text{ seeds/acre}} = 5 \text{ acres} \]
Therefore, the correct answer is 5 acres.