Can anyone tell me does radiation or chemo actually offer a decent quality of life for the limited life expectancy of a stage 4 glioma brain tumor?

Yes: For many patients (not all) surgery, then radiation with chemotherapy, and "maintenance" chemotherapy can give a good quality of life for a period of time for patients with grade IV astrocytomas. Half of all patients with current best therapy for these tumors live over a year, and most of that time should be reasonably good quality unless the tumor has caused irreversible neurological deficits.