Class 4 animals are either suspected of suffering from an infectious disease and highly contagious for other patients, or are suspected of suffering from a zoonotic disease which is easily transmissible to humans (including the vet).
Biosecurity regulations thus aim to minimise the risk of transmission. It is therefore primordially important to implement them as soon as it is suspected, and not only once it has been confirmed, that the patient is suffering from a specific infectious disease!