For what length of time do people with grade 4 terminal cancer have left to live?

Ask his/her doctor. Depends on many factors, including type of stage 4 cancer, current overall health, functional status, ongoing palliative treatments including palliative chemotherapy, and so on. His/her doctors (including a hospice or palliative doctor) could give an average estimate and tell you how likely that estimate would apply to that person's situation.