Sorry about your stage 4 lung cancer. I was diagnosed 4 years ago with stage 1 but it is multifocal, so I am always dealing with a nodule that becomes concerning. I started out with just a little shortness of breath when I got up so I had it checked out and got my diagnosis. However, I had radiation-induced pneumonitis, , and I am now on Oxygen for exertion and sleep. I have pulmonary fibrosis (Interstitial lung disease) as well as lung cancer. Cancer treatments can cause pneumonitis. The pulmonary function tests should identify if you have something like an obstructive or restrictive lung disease. And CT scans pick up if there is fibrosis in the lung. If it is fibrosis, you may be a candidate for an antifibrotic drug to slow down progression.