With stage 4 lung cancer, what is the life expectancy?

Low, less than 5%.: In stage IV lung cancer, the 5 year survival is <5%. This is where the lung cancer has metastasized to anywhere else in the body. Palliative care might be the best option at this point.

Greatly depends: That depends on many factors: the type of cancer, how much it has spread, whether it is responsive to treatment, whether you have received treatment, etc. Etc. It is very difficult to make a prognostication about "life expectancy." stage 4 cancer, of whatever type, can be very difficult. It is important you get all the information you need that is accurate and helpful.

Greatly depends: That depends on many things: type of cancer, the degree of spread, to where it has spread, how much treatment you have received, etc. If you have stage 4 lung cancer, you need to discuss all the aspects of treatment, prognosis, and diagnosis with your doctor, especially the oncologist, or cancer specialist.