Why is fourth baby born with addison disease and other children healthy, ?

Genetic differences: Every child is different- so one may have blue eyes, the another brown, some tall, some short etc. A baby with addison's has a genetic cause- it got genes that the others didn't or had a mutation early in its development. There are many forms of addison's; some are relatively easy to treat, so i hope your baby's form is one of less serious forms. President kennedy & other notables had addison's.