What to do if i have stage 4 lung cancer why do i get fluid in my lungs?

Pleural irritation: This is usually fluid outside the lung but inside the chest cavity which occurs to due the cancer irritating or invading the lining if the chest cavity (or pleura).

From tumors: The cancer masses may bleed or exude fluid making a pleural effusion. Multiple lesions in the lungs may also congest the lungs or create obstructive pneumonia.