Assignment in Eternity

I read this book and about half way through, the assignment was completed but the hero was killed. So I keep reading and we seem to be going off on a tangent that doesn't have any bearing on the previous plot but you never know with science fiction.  The first guys were superspys using their brains to full capacity and these new guys were also using their brains but for time travel purposes. Aha that's it, these guys are going back in time to help the first guy just before he gets blown up. No, I got to the end of the book and these guys lived happily ever after in another time and place.
Turns out this is a collection of four short stories put together by Robert Heinlein in the 1940s,  however in my copy there are only two.

Where are my other two stories?