Additional Study Information: The purpose of this study is to evaluate whether birtamimab plus standard of care will improve survival in subjects with Mayo Stage IV AL amyloidosis. Birtamimab has not yet been approved by the Food and Drug Administration (FDA). During the first phase of the study, the purpose of this study is to evaluate whether birtamimab plus standard of care will improve survival in subjects with Mayo Stage IV AL amyloidosis. For the first phase of the study where neither the researchers nor the study subjects know who is getting the experimental treatment and who is getting a placebo (the double-blind placebo-controlled phase). A computer will by chance assign you to treatment groups in the study. This is called randomization. This is done by chance because no one knows if one study group is better or worse than the other. A separate informed consent form describing the optional Open Label Extension, or OLE, is a phase of a study that occurs after the randomized portion of the trial is completed if the study drug is found to have the potential for benefit of the study will be provided to you if you are eligible for that portion of the study and if you decide that you would like to participate. During the optional OLE portion of the study, the purpose is to evaluate the long-term safety of birtamimab plus standard of care in Mayo Stage IV AL amyloidosis.