To calculate the profit gained by the seller, you can use the following formula:
\[ \text{Profit} = \text{Cost Price} \times \left(\frac{\text{Profit Percentage}}{100}\right) \]
In this case, the cost price (CP) is Rs. 4000 and the profit percentage is 20%.
Now, substitute the values into the formula:
\[ \text{Profit} = 4000 \times \left(\frac{20}{100}\right) \]
Calculating this:
\[ \text{Profit} = 4000 \times 0.2 = 800 \]
Therefore, the profit that the seller gains is Rs. 800.