Could you tell me what is the life expectancy of stage four small cell lung cancer?

Not great, : ....I'm afraid. Without treatment it can be weeks to a few months at best. Treatment can extend it to 6+ months but seldom more than a year. There are exceptions, of course, depends on the age and health of the patient and how much disease there is. If this is about you or a loved one, i wish you the best. Remember none of us can tell for sure how long we have, so live each day fully.