My mother is in stage 4 kidney cancer. She is also on dialysis. What is her life expectancy?

No clear prediction. Each individual case is unique & predictions are very difficult--but from what you describe, the life expectancy is, unfortunately, not long. Large series show that median survival for patients with stage IV disease is 16 to 20 months in contemporary reports, and the five-year survival rate is less than 10 percent for patients with distant metastases. Kidney failure may worsen the prognosis.
Hard to say. But probably longer than you might expect. It really depends on disease burden and overall health. Making these types of predictions is difficult and usually inaccurate. Perhaps focus on what can be done to maximize quality of life right now and possibly explore any reasonable treatments which might be available. I wish you and your mother the best.