Let the cost price of 1 kg be \( C \).
The shopkeeper claims to sell 1 kg at a 4% profit. Therefore, the selling price (S.P.) of 1 kg is:
\[
S.P. = C + 4% \times C = 1.04C.
\]
However, the shopkeeper gives only 800 gm instead of 1 kg, so for every 1 kg sold, the shopkeeper actually gives 800 gm, which costs him:
\[
\text{Cost Price for 800 gm} = \frac{800}{1000} \times C = 0.8C.
\]
Thus, his profit on selling 800 gm for the price of 1 kg is:
\[
\text{Profit} = S.P. - \text{Cost Price for 800 gm} = 1.04C - 0.8C = 0.24C.
\]
The actual profit percentage is:
\[
\text{Profit Percentage} = \frac{\text{Profit}}{\text{Cost Price for 800 gm}} \times 100 = \frac{0.24C}{0.8C} \times 100 = 30%.
\]
Thus, the actual profit percentage is 30%, which corresponds to option (4).