Not to mention that many children(and mothers)died during childbirth.It is never said that Katie Elder only gave birth to her 4 boys.For all we know she might have had more children and they might have died at an earlie age.
Here's another slant - it never says she gave birth to all of her boys. As you so rightly point out, many mothers died in childbirth, leaving motherless children. Katie might have been step-mother to one or more of the older brothers, which might also explain the age gap.
Mrs. C