Rectal swelling of 4cm may be rectal polyp or rectal cancer, which is more serious. Rectal swelling is generally considered as rectal polyp or rectal cancer, the larger the swelling, the greater the probability of cancer, the diameter of more than 2cm about half of the easy to become cancerous and develop into rectal cancer. About half of the rectal masses larger than 2cm in diameter are prone to cancerous transformation and develop into rectal cancer. Rectal cancer manifests as change of defecation habit, feeling of falling in anus, urgency and heaviness, fine stool, frequent stool, etc. When the cancer breaks down and gets infected, pus and blood in stool may appear. The diagnosis of rectal cancer mainly relies on rectal diagnosis, endoscopy and pathological tissue examination. Treatment of rectal cancer is mainly through surgery, including abdominal perineal combined rectal cancer resection, low anterior rectal resection, transabdominal rectal cancer resection and other surgical methods. If the rectal mass is large, one should be vigilant and go to regular hospitals in time to clarify the nature of the mass and carry out treatment as early as possible, so as to prevent the disease from deteriorating and causing serious impact on the body.