Using the gold system how can i tell when i go eom stage 3 to stage 4 emphasema/copd?

GOLD system : Classify COPD patients based on airflow limitation. Pulmonary function test use the capacity of a pat to exhale all inspired air in one second. Fev1 and fvc/fev1 measurements decrease with increasing severity of lung obstruction. Stage 3 severe COPD fev1 30-45% of normal and stage 4 very severe COPD with fevi less than 30% if normal.

COPD Stages: The stages are based on the severity of COPD as assessed by your lung function tests performed by the doctor.