Chemotherapy is intended to slow down the disease progression, but does not cure it.
Long answer: The median survival of someone over 70 with Stage 4 lung cancer is about 2-4 months. The fact that you say it is making him weak is a bad sign and likely puts him on the lower end of that spectrum. Chemotherapy comes in many flavors, some more toxic than others and some more effective than others. I would discuss your father’s goals of his care to get a clearer understanding of he wants “everything done that can be done” or would rather just focus on being comfortable.
Without knowing any of the above, the average survival of a Stage 4 lung cancer patient is about 2-8 months, but this is a great question to ask his medical oncologist who could give a much better estimate.