The net volume would be 4,000,000 - 3,000,000 = 1,000,000 shares.
Why Does Net Volume Matter?
Net volume is a measure of momentum. When net volume is positive, indicating that more shares are being bought than sold, the implication is that the demand for the shares is rising and the stock is hot. When net volume is negative, the implication is that the stock is on a downward trend.