Select the INCORRECT statement.

Blood entering the pulmonary capillaries is deoxygenated blood returning from the systemic circulation.

The partial pressure of carbon dioxide ($p\text{CO}_2$) in this deoxygenated blood is $45 \text{ mm Hg}$. After gas exchange in the alveoli, the oxygenated blood leaves with a $p\text{CO}_2$ of $40 \text{ mm Hg}$. Thus, before oxygenation, the $p\text{CO}_2$ in the pulmonary capillaries is $45 \text{ mm Hg}$. This makes statement (1) incorrect and statement (2) correct.

Oxyhaemoglobin is a stronger acid than deoxyhaemoglobin. The formation of oxyhaemoglobin releases $\textH^{+}$ ions, which slightly lowers the pH of the blood, making statement (3) correct.

High $p\text{CO}_2$ and low pH in metabolically active tissues decrease the affinity of haemoglobin for oxygen. This phenomenon, known as the Bohr effect, facilitates the dissociation of $\text{HbO}_2$ and the release of $\text{O}_2$. Thus, statement (4) is correct..

Answer: The _{pp}$\text{CO}_2$ of blood in pulmonary capillaries is $40$ mm Hg before oxygenation.