Person has stage 4 cancer with mets to the brain, and now has bilateral pneumonia, how bad is this? Is this the cancer trying to spread? What's next?

Very bad. The cancer does not have to try to spread, it already has. Discuss this with the person's doctor and get an estimate of how long they are expected to survive and whether it is time to call in hospice. You also need to ask what further therapy is likely to make a meaningful difference in survival and whether that survival is functional or just further misery. Good luck.