What is the succes rate of curing lung cancer, my friend has it, and it is now at stage 4..?

At a stage 4, not very good. But, these days there are more treatments available than before. So, best to ask the odds from an oncologist.

What is his age? current health status?

Stage 4 cancer is the most advanced cancer stage. We can only hope and pray that he gets well through chemotherapy and radiation-oncology. Tell him to have a tough mental attitude towards the disease. He might overcome it.
Stage IV is very difficult to treat, and the prognosis is usually not good. But please don't give up, stay strong for this person, and have hope.
Here's some links to some helpful information for you:
www.cancer.org
www.cancer.gov
www.cancercare.org
www.lungcancer.org
mother-in-law battling stage IV cervical cancer
His chances of survival are slim at best.
The chances of surviving 5 years for advanced late stage disease which has spread to other parts of the body are very poor, approximately 3%.

This will vary according to you friend's individual situation (race, age, country of treatment, financial situation).
National Cancer Institute statistics search page:
http://seer.cancer.gov/cgi-bin/csr/1975_...