Thanks for writing in.
Its a tough situation for you I guess but you should not loose hope.
Will power is the best medicine cancer patients have to fight longevity.
Coming to your specific question, Lung cancer
stage 4 is medically a stage where the patient pain is alleviated using palliative care
and support. Under optimum conditions, and with lots of support and positive will power, a patient could live for 6 months to a year.
God Bless you and your family.
Hope this helps.