To calculate the area of land needed for planting 20,000 seeds at a density of 4,000 seeds per acre, you can use the following formula:
\[ \text{Area} = \frac{\text{Total seeds}}{\text{Seeds per acre}} \]
Substituting the given numbers:
\[ \text{Area} = \frac{20,000 \text{ seeds}}{4,000 \text{ seeds per acre}} = 5 \text{ acres} \]
Therefore, the correct answer is 5 acres.