Why is beta thalassemia major not lethal while alpha thalassemia with loss of all 4 genes lethal?
This has to do with the fact that fetal hemoglobin (HbF) isn't made of the same chains as adult hemoglobin. The latter comes with 2 alpha chains and 2 beta chains while the main fetal hemoglobin is made of 2 alpha chains and 2 gamma chains. Hence any disease affecting the quality or quantity of beta chains won't have nearly as much impact on a fetus as it does on an adult.

Reference: https://www.uptodate.com/contents/fetal-hemoglobin-hemoglobin-f-in-health-and-disease